U.S. patent application number 13/961293 was filed with the patent office on 2014-02-13 for light-emitting element, light-emitting device, display device, electronic device, and lighting device.
This patent application is currently assigned to Semiconductor Energy Laboratory Co., Ltd.. The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Takahiro Ishisone, Satoshi Seo.
Application Number | 20140042469 13/961293 |
Document ID | / |
Family ID | 50050853 |
Filed Date | 2014-02-13 |
United States Patent
Application |
20140042469 |
Kind Code |
A1 |
Seo; Satoshi ; et
al. |
February 13, 2014 |
Light-Emitting Element, Light-Emitting Device, Display Device,
Electronic Device, and Lighting Device
Abstract
A light-emitting element which uses a plurality of kinds of
light-emitting dopants emitting light in a balanced manner and has
high emission efficiency is provided. Further, a light-emitting
device, a display device, an electronic device, and a lighting
device each having reduced power consumption by using the above
light-emitting element are provided. A light-emitting element which
includes a plurality of light-emitting layers including different
phosphorescent materials is provided. In the light-emitting
element, the light-emitting layer which includes a light-emitting
material emitting light with a long wavelength includes two kinds
of carrier-transport compounds having properties of transporting
carriers with different polarities. Further, in the light-emitting
element, the triplet excitation energy of a host material included
in the light-emitting layer emitting light with a short wavelength
is higher than the triplet excitation energy of at least one of the
carrier-transport compounds.
Inventors: |
Seo; Satoshi; (Sagamihara,
JP) ; Ishisone; Takahiro; (Atsugi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Kanagawa-ken |
|
JP |
|
|
Assignee: |
Semiconductor Energy Laboratory
Co., Ltd.
Kanagawa-ken
JP
|
Family ID: |
50050853 |
Appl. No.: |
13/961293 |
Filed: |
August 7, 2013 |
Current U.S.
Class: |
257/94 |
Current CPC
Class: |
H01L 27/322 20130101;
H01L 2251/552 20130101; H01L 51/5206 20130101; H01L 27/3213
20130101; H01L 51/5016 20130101; H01L 51/5004 20130101; H01L 51/504
20130101; H01L 33/002 20130101; H01L 51/5072 20130101; H01L 51/5221
20130101; H01L 2251/5384 20130101; H01L 51/5056 20130101 |
Class at
Publication: |
257/94 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 10, 2012 |
JP |
2012-177795 |
Claims
1. A light-emitting device comprising: a first light-emitting layer
between an anode and a cathode, the first light-emitting layer
comprising a first phosphorescent compound and a first host
material; and a second light-emitting layer in contact with the
first light-emitting layer, the second light-emitting layer
comprising a second phosphorescent compound, a first
electron-transport compound, and a first hole-transport compound,
wherein an emission wavelength of the second phosphorescent
compound is longer than an emission wavelength of the first
phosphorescent compound, and wherein a triplet excitation energy of
the first host material is higher than or equal to a triplet
excitation energy of the first electron-transport compound or the
first hole-transport compound.
2. The light-emitting device according to claim 1, wherein the
first electron-transport compound and the first hole-transport
compound form an exciplex.
3. The light-emitting device according to claim 1, wherein the
first host material has an electron-transport property, and wherein
the first light-emitting layer is closer to the cathode than the
second light-emitting layer.
4. The light-emitting device according to claim 1, wherein the
first host material has a hole-transport property, and wherein the
first light-emitting layer is closer to the anode than the second
light-emitting layer.
5. The light-emitting device according to claim 3, wherein the
first light-emitting layer further comprises a second host
material, wherein the second host material has a hole-transport
property, and wherein a triplet excitation energy of the second
host material is higher than or equal to the triplet excitation
energy of the first electron-transport compound or the first
hole-transport compound.
6. The light-emitting device according to claim 5, wherein the
first host material and the second host material form an
exciplex.
7. The light-emitting device according to claim 4, wherein the
first light-emitting layer further comprises a second host
material, wherein the second host material has an
electron-transport property, and wherein a triplet excitation
energy of the second host material is higher than or equal to the
triplet excitation energy of the first electron-transport compound
or the first hole-transport compound.
8. The light-emitting device according to claim 6, wherein the
first host material and the second host material form an
exciplex.
9. A lighting device comprising the light-emitting device according
to claim 1.
10. An electronic device comprising the light-emitting device
according to claim 1.
11. A light-emitting device comprising: a first light-emitting
layer between an anode and a cathode, the first light-emitting
layer comprising a first phosphorescent compound and a first host
material; a second light-emitting layer in contact with the first
light-emitting layer, the second light-emitting layer comprising a
second phosphorescent compound, a first electron-transport
compound, and a first hole-transport compound; and a third
light-emitting layer in contact with the second light-emitting
layer, the third light-emitting layer comprising a third
phosphorescent compound, a second electron-transport compound, and
a second hole-transport compound, wherein an emission wavelength of
the second phosphorescent compound is longer than an emission
wavelength of the first phosphorescent compound, wherein an
emission wavelength of the third phosphorescent compound is longer
than the emission wavelength of the second phosphorescent compound,
and wherein a triplet excitation energy of the first host material
is higher than or equal to a triplet excitation energy of the first
electron-transport compound or the first hole-transport
compound.
12. The light-emitting device according to claim 11, wherein the
triplet excitation energies of the first electron-transport
compound and the first hole-transport compound are higher than a
triplet excitation energy of the second electron-transport compound
or the second hole-transport compound.
13. The light-emitting device according to claim 11, wherein the
first electron-transport compound and the first hole-transport
compound form an exciplex, and wherein the second
electron-transport compound and the second hole-transport compound
form an exciplex.
14. The light-emitting device according to claim 11, wherein the
first host material has an electron-transport property, and wherein
the first light-emitting layer is closer to the cathode than the
second light-emitting layer.
15. The light-emitting device according to claim 11, wherein the
first host material has a hole-transport property, and wherein the
first light-emitting layer is closer to the anode than the second
light-emitting layer.
16. The light-emitting device according to claim 14, wherein the
first light-emitting layer further comprises a second host
material, wherein the second host material has a hole-transport
property, and wherein a triplet excitation energy of the second
host material is higher than or equal to the triplet excitation
energy of the first electron-transport compound or the first
hole-transport compound.
17. The light-emitting device according to claim 16, wherein the
first host material and the second host material form an
exciplex.
18. The light-emitting device according to claim 15, wherein the
first light-emitting layer further comprises a second host
material, wherein the second host material has an
electron-transport property, and wherein a triplet excitation
energy of the second host material is higher than or equal to the
triplet excitation energy of the first electron-transport compound
or the first hole-transport compound.
19. The light-emitting device according to claim 18, wherein the
first host material and the second host material form an
exciplex.
20. The light-emitting device according to claim 11, wherein the
first electron-transport compound and the second electron-transport
compound are the same material.
21. The light-emitting device according to claim 11, wherein the
first hole-transport compound and the second hole-transport
compound are the same material.
22. The light-emitting device according to claim 11, wherein the
first electron-transport compound and the second electron-transport
compound are the same material, and wherein the first
hole-transport compound and the second hole-transport compound are
the same material.
23. A lighting device comprising the light-emitting device
according to claim 11.
24. An electronic device comprising the light-emitting device
according to claim 11.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a light-emitting element, a
display device, a light-emitting device, an electronic device, and
a lighting device each of which includes an organic compound as a
light-emitting substance.
[0003] 2. Description of the Related Art
[0004] In recent years, research and development have been
extensively conducted on light-emitting elements using
electroluminescence (EL). In a basic structure of such a
light-emitting element, a layer containing a light-emitting
substance (an EL layer) is interposed between a pair of electrodes.
By applying voltage to this element, light emission from the
light-emitting substance can be obtained.
[0005] Since such a light-emitting element is of
self-light-emitting type, the light-emitting element has advantages
over a liquid crystal display in that visibility of pixels is high,
a backlight is not required, and so on and is therefore thought to
be suitable as flat panel display elements. In addition, it is also
a great advantage that a display including such a light-emitting
element can be manufactured as a thin and lightweight display.
Furthermore, very high speed response is also one of the features
of such an element.
[0006] Since a light-emitting layer of such a light-emitting
element can be formed in the form of a film, planar light emission
can be achieved. Therefore, large-area elements can be easily
formed. This feature is difficult to obtain with point light
sources typified by incandescent lamps and LEDs or linear light
sources typified by fluorescent lamps. Thus, light-emitting
elements also have great potential as planar light sources
applicable to lighting devices and the like.
[0007] In the case of an organic EL element in which an EL layer
containing an organic compound as the light-emitting substance is
provided between a pair of electrodes, application of a voltage
between the pair of electrodes causes injection of electrons from
the cathode and holes from the anode into the EL layer having a
light-emitting property, and thus a current flows. By recombination
of the injected electrons and holes, the organic compound having a
light-emitting property is excited and provides light emission from
the excited state.
[0008] The excited state of an organic compound can be a singlet
excited state or a triplet excited state, and light emission from
the singlet excited state (S*) is referred to as fluorescence, and
light emission from the triplet excited state (T*) is referred to
as phosphorescence. The statistical generation ratio of the excited
states in the light-emitting element is considered to be
S*:T*=1:3.
[0009] In a compound that emits light from the singlet excited
state (hereinafter, referred to as a fluorescent compound), at room
temperature, generally light emission from the triplet excited
state (phosphorescence) is not observed while only light emission
from the singlet excited state (fluorescence) is observed.
Therefore, the internal quantum efficiency (the ratio of generated
photons to injected carriers) of a light-emitting element using a
fluorescent compound is assumed to have a theoretical limit of 25%
based on the ratio of S* to T* which is 1:3.
[0010] In contrast, in a compound that emits light from the triplet
excited state (hereinafter, referred to as a phosphorescent
compound), light emission from the triplet excited state
(phosphorescence) is observed. Further, in a phosphorescent
compound, since intersystem crossing (i.e., transfer from a singlet
excited state to a triplet excited state) easily occurs, the
internal quantum efficiency can be increased to 100% in theory.
That is, higher emission efficiency can be achieved than using a
fluorescent compound. For this reason, light-emitting elements
using phosphorescent compounds are now under active development in
order to obtain highly efficient light-emitting elements.
[0011] A white light-emitting element disclosed in Patent Document
1 includes a light-emitting region containing a plurality of kinds
of light-emitting dopants which emit phosphorescence.
REFERENCE
Patent Document
[0012] [Patent Document 1] Japanese Translation of PCT
International Application No. 2004-522276
SUMMARY OF THE INVENTION
[0013] Although an internal quantum efficiency of 100% in a
phosphorescent compound is theoretically possible, such high
efficiency can be hardly achieved without optimization of an
element structure or a combination with another material.
Especially in a light-emitting element which includes a plurality
of kinds of phosphorescent compounds having different bands
(different emission colors) as light-emitting dopants, it is
difficult to obtain highly efficient light emission without not
only considering energy transfer but also optimizing the efficiency
of the energy transfer. In fact, in Patent Document 1, even when
all the light-emitting dopants of a light-emitting element are
phosphorescent compounds, the external quantum efficiency is
approximately 3% to 4%. It is thus presumed that even when light
extraction efficiency is taken into account, the internal quantum
efficiency is 20% or lower, which is low for a phosphorescent
light-emitting element.
[0014] In a multicolor light-emitting element using dopants
exhibiting different emission colors, beside improvement of
emission efficiency, it is also necessary to attain a good balance
between light emissions by the dopants which exhibit different
emission colors. It is not easy to keep a balance between light
emissions by the dopants and to achieve high emission efficiency at
the same time.
[0015] In view of the above, an object of one embodiment of the
present invention is to provide a light-emitting element which uses
a plurality of kinds of light-emitting dopants emitting light in a
balanced manner and has high emission efficiency. Another object of
one embodiment of the present invention is to provide a
light-emitting device, a display device, an electronic device, and
a lighting device each having reduced power consumption by using
the above light-emitting element.
[0016] It is only necessary that at least one of the
above-described objects be achieved in the present invention.
[0017] According to the present invention, a light-emitting element
which includes a plurality of light-emitting layers including
different phosphorescent materials is provided. The light-emitting
layer of the plurality of light-emitting layers which includes a
light-emitting material emitting light with a long wavelength
includes two kinds of carrier-transport compounds having properties
of transporting carriers with different polarities. Further, the
light-emitting layer of the plurality of light-emitting layers
which includes a light-emitting material emitting light with a
short wavelength includes a host material, and the triplet
excitation energy of the host material is higher than the triplet
excitation energy of at least one of the carrier-transport
compounds. By such a combination of a host material and a
carrier-transport compound with which energy can be transferred
efficiently between hosts, a light-emitting element of one
embodiment of the present invention can be provided.
[0018] That is, one embodiment of the present invention is a
light-emitting element including: a first light-emitting layer
including a first phosphorescent compound and a first host
material, between an anode and a cathode; and a second
light-emitting layer including a second phosphorescent compound, a
first electron-transport compound, and a first hole-transport
compound, between the anode and the cathode. An emission wavelength
of the second phosphorescent compound is longer than an emission
wavelength of the first phosphorescent compound. A triplet
excitation energy of the first host material is higher than or
equal to a triplet excitation energy of the first
electron-transport compound or the first hole-transport compound.
The first light-emitting layer and the second light-emitting layer
are in contact with each other.
[0019] Another embodiment of the present invention is the
light-emitting element in which the first electron-transport
compound and the first hole-transport compound form an
exciplex.
[0020] Another embodiment of the present invention is the
light-emitting element in which the first host material is an
electron-transport compound, and in which the first light-emitting
layer is closer to the cathode than the second light-emitting
layer.
[0021] Another embodiment of the present invention is the
light-emitting element in which the first host material is a
hole-transport compound, and in which the first light-emitting
layer is closer to the anode than the second light-emitting
layer.
[0022] Another embodiment of the present invention is the
light-emitting element in which the first light-emitting layer
further includes a second host material, in which the second host
material is a hole-transport compound, and in which a triplet
excitation energy of the second host material is higher than or
equal to the triplet excitation energy of the first
electron-transport compound or the first hole-transport
compound.
[0023] Another embodiment of the present invention is the
light-emitting element in which the first light-emitting layer
further includes a second host material, in which the second host
material is an electron-transport compound, and in which a triplet
excitation energy of the second host material is higher than or
equal to the triplet excitation energy of the first
electron-transport compound or the first hole-transport
compound.
[0024] Another embodiment of the present invention is the
light-emitting element in which the first host material and the
second host material form an exciplex.
[0025] Another embodiment of the present invention is a
light-emitting element including: a first light-emitting layer
including a first phosphorescent compound and a first host
material, between an anode and a cathode; a second light-emitting
layer including a second phosphorescent compound, a first
electron-transport compound, and a first hole-transport compound,
between the anode and the cathode; and a third light-emitting layer
including a third phosphorescent compound, a second
electron-transport compound, and a second hole-transport compound,
between the anode and the cathode. An emission wavelength of the
second phosphorescent compound is longer than an emission
wavelength of the first phosphorescent compound. An emission
wavelength of the third phosphorescent compound is longer than the
emission wavelength of the second phosphorescent compound. A
triplet excitation energy of the first host material is higher than
or equal to a triplet excitation energy of the first
electron-transport compound or the first hole-transport compound.
The second light-emitting layer is in contact with the first
light-emitting layer, and the third light-emitting layer is in
contact with the second light-emitting layer.
[0026] Another embodiment of the present invention is a
light-emitting element including: a first light-emitting layer
including a first phosphorescent compound and a first host
material, between an anode and a cathode; a second light-emitting
layer including a second phosphorescent compound, a first
electron-transport compound, and a first hole-transport compound,
between the anode and the cathode; and a third light-emitting layer
including a third phosphorescent compound, a second
electron-transport compound, and a second hole-transport compound,
between the anode and the cathode. An emission wavelength of the
second phosphorescent compound is longer than an emission
wavelength of the first phosphorescent compound. An emission
wavelength of the third phosphorescent compound is longer than the
emission wavelength of the second phosphorescent compound. A
triplet excitation energy of the first host material is higher than
or equal to a triplet excitation energy of the first
electron-transport compound or the first hole-transport compound.
The triplet excitation energies of the first electron-transport
compound and the first hole-transport compound are higher than a
triplet excitation energy of the second electron-transport compound
or the second hole-transport compound. The second light-emitting
layer is in contact with the first light-emitting layer, and the
third light-emitting layer is in contact with the second
light-emitting layer.
[0027] Another embodiment of the present invention is the
light-emitting element in which the first electron-transport
compound and the first hole-transport compound form an exciplex,
and in which the second electron-transport compound and the second
hole-transport compound form an exciplex.
[0028] Another embodiment of the present invention is the
light-emitting element in which the first host material is an
electron-transport compound, and in which the first light-emitting
layer is closer to the cathode than the second light-emitting
layer.
[0029] Another embodiment of the present invention is the
light-emitting element in which the first host material is a
hole-transport compound, and in which the first light-emitting
layer is closer to the anode than the second light-emitting
layer.
[0030] Another embodiment of the present invention is the
light-emitting element in which the first light-emitting layer
further includes a second host material, in which the second host
material is a hole-transport compound, and in which a triplet
excitation energy of the second host material is higher than or
equal to the triplet excitation energy of the first
electron-transport compound or the first hole-transport
compound.
[0031] Another embodiment of the present invention is the
light-emitting element in which the first light-emitting layer
further includes a second host material, in which the second host
material is an electron-transport compound, and in which a triplet
excitation energy of the second host material is higher than or
equal to the triplet excitation energy of the first
electron-transport compound or the first hole-transport
compound.
[0032] Another embodiment of the present invention is the
light-emitting element in which the first host material and the
second host material form an exciplex.
[0033] Another embodiment of the present invention is the
light-emitting element in which the first electron-transport
compound and the second electron-transport compound are the same
material.
[0034] Another embodiment of the present invention is the
light-emitting element in which the first hole-transport compound
and the second hole-transport compound are the same material.
[0035] Another embodiment of the present invention is the
light-emitting element in which the first electron-transport
compound and the second electron-transport compound are the same
material, and in which the first hole-transport compound and the
second hole-transport compound are the same material.
[0036] Another embodiment of the present invention is the
light-emitting element in which a thickness of the second
light-emitting layer is greater than or equal to 2 nm and less than
or equal to 20 nm.
[0037] Another embodiment of the present invention is the
light-emitting element in which the thickness of the second
light-emitting layer is greater than or equal to 5 nm and less than
or equal to 10 nm.
[0038] Another embodiment of the present invention is a
light-emitting device, a light-emitting display device, an
electronic device, and a lighting device each including the
light-emitting element.
[0039] Note that the light-emitting device in this specification
includes, in its category, an image display device with a
light-emitting element. Further, the category of the light-emitting
device in this specification includes a module in which a
light-emitting device is provided with a connector such as an
anisotropic conductive film or a TCP (tape carrier package); a
module in which the end of the TCP is provided with a printed
wiring board; and a module in which an IC (integrated circuit) is
directly mounted on a light-emitting device by a COG (chip on
glass) method. Furthermore, the category includes light-emitting
devices that are used in lighting equipment or the like.
[0040] One embodiment of the present invention can provide a
light-emitting element having high emission efficiency. By using
the light-emitting element, another embodiment of the present
invention can provide any of a light-emitting device, a
light-emitting display device, an electronic device, and a lighting
device each having reduced power consumption.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIGS. 1A, 1B, 1C, 1D, and 1E are schematic diagrams of
light-emitting elements.
[0042] FIG. 2 is a schematic diagram of a light-emitting
element.
[0043] FIGS. 3A and 3B are schematic diagrams of an active matrix
light-emitting device.
[0044] FIGS. 4A and 4B are schematic diagrams of a passive matrix
light-emitting device.
[0045] FIGS. 5A and 5B are each a schematic diagram of an active
matrix light-emitting device.
[0046] FIG. 6 is a schematic diagram of an active matrix
light-emitting device.
[0047] FIGS. 7A and 7B are schematic diagrams of a lighting
device.
[0048] FIGS. 8A, 8B1, 8B2, 8C, and 8D illustrate electronic
devices.
[0049] FIG. 9 illustrates an electronic device.
[0050] FIG. 10 illustrates a lighting device.
[0051] FIG. 11 illustrates a lighting device.
[0052] FIG. 12 illustrates in-vehicle display devices and lighting
devices.
[0053] FIGS. 13A to 13C illustrate an electronic device.
[0054] FIG. 14 shows an emission spectrum of a light-emitting
element 1.
[0055] FIG. 15 shows luminance-current efficiency characteristics
of the light-emitting element 1.
[0056] FIG. 16 shows luminance-external quantum efficiency
characteristics of the light-emitting element 1.
[0057] FIG. 17 shows voltage-luminance characteristics of the
light-emitting element 1.
[0058] FIG. 18 shows luminance-power efficiency characteristics of
the light-emitting element 1.
[0059] FIG. 19 shows emission spectra of a light-emitting element 2
and a light-emitting element 3.
[0060] FIG. 20 shows luminance-current efficiency characteristics
of the light-emitting element 2 and the light-emitting element
3.
[0061] FIG. 21 shows luminance-external quantum efficiency
characteristics of the light-emitting element 2 and the
light-emitting element 3.
[0062] FIG. 22 shows voltage-luminance characteristics of the
light-emitting element 2 and the light-emitting element 3.
[0063] FIG. 23 shows luminance-power efficiency characteristics of
the light-emitting element 2 and the light-emitting element 3.
[0064] FIG. 24 shows an emission spectrum of a light-emitting
element 4.
[0065] FIG. 25 shows luminance-current efficiency characteristics
of the light-emitting element 4.
[0066] FIG. 26 shows luminance-external quantum efficiency
characteristics of the light-emitting element 4.
[0067] FIG. 27 shows voltage-luminance characteristics of the
light-emitting element 4.
[0068] FIG. 28 shows luminance-power efficiency characteristics of
the light-emitting element 4.
[0069] FIG. 29 shows time dependence of normalized luminance of the
light-emitting element 4.
[0070] FIG. 30 shows a phosphorescent spectrum of 35DCzPPy.
[0071] FIG. 31 shows a phosphorescent spectrum of PCCP.
[0072] FIG. 32 shows a phosphorescent spectrum of 2mDBTPDBq-II.
[0073] FIG. 33 shows a phosphorescent spectrum of PCBA1BP.
[0074] FIG. 34 shows a phosphorescent spectrum of
2mDBTBPDBq-II.
[0075] FIG. 35 shows a phosphorescent spectrum of PCBNBB.
DETAILED DESCRIPTION OF THE INVENTION
[0076] Hereinafter, embodiments and examples of the present
invention will be described in detail with reference to the
accompanying drawings. Note that the present invention is not
limited to the following description, and it will be easily
understood by those skilled in the art that various changes and
modifications can be made without departing from the spirit and
scope of the present invention. Therefore, the present invention
should not be construed as being limited to the description in the
following embodiments and examples.
Embodiment 1
[0077] First, the operation principle of a light-emitting element
of one embodiment of the present invention will be described. The
point of the present invention is that a first phosphorescent
compound and a second phosphorescent compound emitting light whose
wavelength is longer than that of light emitted from the first
phosphorescent compound are used and both of the first and second
phosphorescent compounds are made to emit light efficiently,
whereby a multicolor light-emitting element with high efficiency is
obtained.
[0078] As a general method for obtaining a multicolor
light-emitting element including a phosphorescent compound, a
method can be given in which a plurality of kinds of phosphorescent
compounds having different emission colors are dispersed in some
host material in an appropriate ratio. However, in such a method,
the phosphorescent compound which emits light with the longest
wavelength readily emits light, so that it is extremely difficult
to design and control an element structure (especially the
concentrations of the phosphorescent compounds in the host
material) for obtaining polychromatic light.
[0079] As another technique for obtaining a multicolor
light-emitting element, what is called a tandem structure, in which
light-emitting elements having different emission colors are
stacked in series, can be given. For example, a blue light-emitting
element, a green light-emitting element, and a red light-emitting
element are stacked in series and made to emit light at the same
time, whereby polychromatic light (in this case, white light) can
be easily obtained. The element structure can be relatively easily
designed and controlled because the blue light-emitting element,
the green light-emitting element, and the red light-emitting
element can be independently optimized. However, the stacking of
three elements is accompanied by an increase in the number of
layers and makes the fabrication complicated. In addition, when a
problem occurs in electrical contact at connection portions between
the elements (what is called intermediate layers), an increase in
drive voltage, i.e., power loss might be caused.
[0080] In contrast, in a light-emitting element of one embodiment
of the present invention, a first light-emitting layer and a second
light-emitting layer are stacked between a pair of electrodes. The
first light-emitting layer includes a first phosphorescent compound
and a first host material. The second light-emitting layer includes
a second phosphorescent compound emitting light whose wavelength is
longer than that of the first phosphorescent compound, a first
electron-transport compound, and a first hole-transport compound.
Here, the triplet excitation energy of the first host material is
higher than that of the first electron-transport compound or the
first hole-transport compound, and the first light-emitting layer
and the second light-emitting layer are provided in contact with
each other unlike in a tandem structure.
[0081] FIG. 1E schematically illustrates the element structure of
the light-emitting element of one embodiment of the present
invention. In FIG. 1E, a first electrode 101, a second electrode
102, and an EL layer 103 are illustrated. The EL layer 103 includes
at least a light-emitting layer 113 and other layers may be
provided as appropriate. In the structure illustrated in FIG. 1E, a
hole-injection layer 111, a hole-transport layer 112, an
electron-transport layer 114, and an electron-injection layer 115
are assumed to be provided. Note that it is assumed that the first
electrode 101 functions as an anode and the second electrode 102
functions as a cathode.
[0082] FIGS. 1A and 1B are each an enlarged view of the
light-emitting layer 113 in the light-emitting element. In each of
FIGS. 1A and 1B, a first light-emitting layer 113a, a second
light-emitting layer 113b, the light-emitting layer 113 which is a
combination of the two layers, a first phosphorescent compound
113Da, a second phosphorescent compound 113Db, a first host
material 113Ha1, a first carrier-transport compound 113H.sub.1, and
a second carrier-transport compound 113H.sub.2 are illustrated.
FIG. 1B is a schematic diagram illustrating the case where the
first light-emitting layer 113a further includes a second host
material 113Ha2. Note that the first host material 113Ha1 and the
first carrier-transport compound 113H.sub.1 may be the same or
different from each other; and the second host material 113Ha2 and
the second carrier-transport compound 113H.sub.2 may be the same or
different from each other. The first light-emitting layer 113a may
be on the anode side and the second light-emitting layer 113b may
be on the cathode side, or the first light-emitting layer 113a may
be on the cathode side and the second light-emitting layer 113b may
be on the anode side. Note that one of the first host material
113Ha1 and the second host material 113Ha2 is an electron-transport
compound, and the other of them is a hole-transport compound.
Similarly, one of the first carrier-transport compound 113H.sub.1
and the second carrier-transport compound 113H.sub.2 is an
electron-transport compound, and the other of them is a
hole-transport compound.
[0083] The position of a recombination region in the light-emitting
layer can be adjusted with the mixture ratio of the first host
material 113Ha1 to the second host material 113Ha2, the first
carrier-transport compound 113H.sub.1, and the second
carrier-transport compound 113H.sub.2 which are included in the
light-emitting layers. As described above, one of the first host
material 113Ha1 and the second host material 113Ha2 is an
electron-transport compound, and the other of them is a
hole-transport compound; and one of the first carrier-transport
compound 113H.sub.1 and the second carrier-transport compound
113H.sub.2 is an electron-transport compound, and the other of them
is a hole-transport compound. For such a reason, changing the
mixture ratio thereof can adjust the carrier-transport property of
each light-emitting layer, and accordingly can easily control the
position of the recombination region.
[0084] Note that light emission from the first phosphorescent
compound 113Da is difficult to obtain in the case where an exciton
is directly generated in the second light-emitting layer 113b;
therefore, the carrier recombination region is preferably in the
first light-emitting layer 113a or in the vicinity of the interface
between the first light-emitting layer 113a and the second
light-emitting layer 113b.
[0085] In order that the carrier recombination region is in the
vicinity of the interface between the first light-emitting layer
113a and the second light-emitting layer 113b, the first
light-emitting layer 113a is made to have a hole-transport property
in the case where the first light-emitting layer 113a is on the
anode side, or is made to have an electron-transport property in
the case where the first light-emitting layer 113a is on the
cathode side. Further, the second light-emitting layer 113b is made
to have an opposite carrier-transport property to that of the first
light-emitting layer 113a; thus, the recombination region can be
made in the vicinity of the interface between the first
light-emitting layer 113a and the second light-emitting layer 113b.
In order that the carrier recombination region is in the first
light-emitting layer 113a, the bipolar property of the first
light-emitting layer 113a is improved with the above structure as a
base.
[0086] In the case of the structure illustrated in FIG. 1A, the
first host material 113Ha1 is made to have a hole-transport
property in the case where the first light-emitting layer 113a is
on the anode side, or is made to have an electron-transport
property in the case where the first light-emitting layer 113a is
on the cathode side; the second light-emitting layer 113b is made
to have an opposite carrier-transport property to that of the first
light-emitting layer 113a by changing the mixture ratio of the
first carrier-transport compound 113H.sub.1 to the second
carrier-transport compound 113H.sub.2.
[0087] In the case of a combination of the second light-emitting
layer 113b and the first light-emitting layer 113a illustrated in
FIG. 1C, the carrier-transport properties of the first
light-emitting layer 113a and the second light-emitting layer 113b
can be adjusted by changing the mixture ratio of the
electron-transport compound to the hole-transport compound in each
of the light-emitting layers.
[0088] Note that when the recombination region is in the first
light-emitting layer 113a or at the interface between the first
light-emitting layer and the second light-emitting layer, the
intensity of light emission from the second phosphorescent compound
113Db is lower than that of light emission from the second
phosphorescent compound 113Da in some cases. In view of this, in
one embodiment of the present invention, a combination of materials
is selected such that the triplet excitation energy of the first
host material 113Ha1 is higher than that of the first
carrier-transport compound 113H.sub.1 and/or that of the second
carrier-transport compound 113H.sub.2 in the case of the structure
illustrated in FIG. 1A. The triplet excitation energy due to
recombination of carriers partly moves from the triplet excitation
level of the first host material 113Ha1 to that of the first
carrier-transport compound 113H.sub.1 and/or that of the second
carrier-transport compound 113H.sub.2; in this manner, the second
phosphorescent compound 113Db can emit light.
[0089] In the case where the first light-emitting layer 113a
further includes the second host material 113Ha2 as illustrated in
FIG. 1B, a combination of materials is selected such that the
triplet excitation energy of the second host material 113Ha2 is
higher than that of the first carrier-transport compound 113H.sub.1
and/or that of the second carrier-transport compound 113H.sub.2.
The triplet excitation energy due to recombination of carriers
partly moves from the triplet excitation level of the second host
material 113Ha2 to that of the first carrier-transport compound
113H.sub.1 and/or that of the second carrier-transport compound
113H.sub.2; in this manner, the second phosphorescent compound
113Db can emit light.
[0090] In this manner, the transfer of triplet excitation energy
which accounts for 75% of the excitation energy generated by
recombination of carriers is taken into consideration; thus, light
emission from the second phosphorescent compound 113Db can be
obtained with desired intensity.
[0091] In the case where the singlet excitation energy of the first
host material 113Ha1 or the second host material 113Ha2 is higher
than the single excitation energies of the first carrier-transport
compound 113H.sub.1 and the second carrier-transport compound
113H.sub.2, energy transfer occurs due to Dexter mechanism. Here,
when the first host material 113Ha1 or the second host material
113Ha2 is a fluorescent light-emitting material, energy transfer
occurs also owing to Forster mechanism.
[0092] Here, energy transfer to the phosphorescent compound for
obtaining a light-emitting element having higher emission
efficiency will be described. In the following description, a
substance providing a phosphorescent compound with energy is
referred to as a host material.
[0093] Carrier recombination occurs in both the host material and
the phosphorescent compound; thus, efficient energy transfer from
the host material to the phosphorescent compound is needed to
increase emission efficiency. As mechanisms of the energy transfer
from the host material to the phosphorescent compound, two
mechanisms have been proposed: one is Dexter mechanism, and the
other is Forster mechanism.
[0094] The efficiency of energy transfer from a host molecule to a
guest molecule (energy transfer efficiency .PHI..sub.ET) is
expressed by the following formula. In the formula, k.sub.r denotes
the rate constant of a light-emission process (fluorescence in
energy transfer from a singlet excited state, and phosphorescence
in energy transfer from a triplet excited state), k.sub.n denotes
the rate constant of a non-light-emission process (thermal
deactivation or intersystem crossing), and .tau. denotes the
measured lifetime of an excited state.
.PHI. ET = k h * .fwdarw. g k r + k n + k h * .fwdarw. g = k h *
.fwdarw. g ( 1 .tau. ) + k h * .fwdarw. g [ Formula 1 ]
##EQU00001##
[0095] First, according to the above formula, it is understood that
the energy transfer efficiency .PHI..sub.ET can be increased by
significantly increasing the rate constant k.sub.h*.fwdarw.g of
energy transfer as compared with another competing rate constant
k.sub.r+k.sub.n(=1/.tau.). Then, in order to increase the rate
constant k.sub.h*.fwdarw.g of energy transfer, in Forster mechanism
and Dexter mechanism, it is preferable that an emission spectrum of
a host molecule (a fluorescent spectrum in energy transfer from a
singlet excited state, and a phosphorescent spectrum in energy
transfer from a triplet excited state) largely overlap with an
absorption spectrum of a guest molecule (the phosphorescent
compound in the second light-emitting layer).
[0096] Here, the absorption band on the longest wavelength side
(lowest energy side) in the absorption spectrum of the
phosphorescent compound is of importance in considering the overlap
between the emission spectrum of the host molecule and the
absorption spectrum of the phosphorescent compound.
[0097] In an absorption spectrum of the phosphorescent compound, an
absorption band that is considered to contribute to light emission
most greatly is at an absorption wavelength corresponding to direct
transition from a ground state to a triplet excitation state and a
vicinity of the absorption wavelength, which is on the longest
wavelength side. From these considerations, it is preferable that
the emission spectrum (a fluorescent spectrum and a phosphorescent
spectrum) of the host material overlap with the absorption band on
the longest wavelength side in the absorption spectrum of the
phosphorescent compound.
[0098] For example, most organometallic complexes, especially
light-emitting iridium complexes, have a broad absorption band at
around 500 nm to 600 nm as the absorption band on the longest
wavelength side. This absorption band is mainly based on a triplet
MLCT (metal to ligand charge transfer) transition. Note that it is
considered that the absorption band also includes absorptions based
on a triplet .pi.-.pi.* transition and a singlet MLCT transition,
and that these absorptions overlap with each other to form a broad
absorption band on the longest wavelength side in the absorption
spectrum. Therefore, when an organometallic complex (especially
iridium complex) is used as the guest material, it is preferable to
make the broad absorption band on the longest wavelength side
largely overlap with the emission spectrum of the host material as
described above.
[0099] Here, first, energy transfer from a host material in a
triplet excited state will be considered. From the above-described
discussion, it is preferable that, in energy transfer from a
triplet excited state, the phosphorescent spectrum of the host
material and the absorption band on the longest wavelength side of
the phosphorescent compound largely overlap with each other.
[0100] However, a question here is energy transfer from the host
molecule in the singlet excited state. In order to efficiently
perform not only energy transfer from the triplet excited state but
also energy transfer from the singlet excited state, it is clear
from the above-described discussion that the host material needs to
be designed such that not only its phosphorescent spectrum but also
its fluorescent spectrum overlaps with the absorption band on the
longest wavelength side of the guest material. In other words,
unless the host material is designed so as to have its fluorescent
spectrum in a position similar to that of its phosphorescent
spectrum, it is not possible to achieve efficient energy transfer
from the host material in both the singlet excited state and the
triplet excited state.
[0101] However, in general, the singlet excitation level differs
greatly from the triplet excitation level (singlet excitation
level>triplet excitation level); therefore, the fluorescence
wavelength also differs greatly from the phosphorescence wavelength
(fluorescence wavelength<phosphorescence wavelength). For
example, 4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP), which
is commonly used in a light-emitting element including a
phosphorescent compound, has a phosphorescent spectrum at around
500 nm and has a fluorescent spectrum at around 400 nm, which are
largely different by about 100 nm. This example also shows that it
is extremely difficult to design a host material so as to have its
fluorescent spectrum in a position similar to that of its
phosphorescent spectrum.
[0102] The singlet excitation energy level of one substance is
higher than the triplet excitation energy level thereof, and thus
the triplet excitation level of a host material whose fluorescence
spectrum corresponds to a wavelength close to an absorption
spectrum of a guest material on the longest wavelength side is
lower than the triplet excitation level of the guest material.
[0103] Here, an exciplex formed from two kinds of materials is
described. Fluorescence from the exciplex is derived from the
energy difference between the higher HOMO level of one of the two
kinds of materials and the lower LUMO level of the other material;
thus, the fluorescence spectrum of the exciplex is on the longer
wavelength side than that of either one of the two kinds of
materials. For such a reason, energy transfer from a single excited
state can be maximized while the triplet excitation levels of the
two kinds of compounds forming the exciplex are kept higher than
the triplet excitation level of the guest material.
[0104] In addition, the exciplex is in a state where the triplet
excitation level and the singlet excitation level are close to each
other; therefore, the fluorescence spectrum and the phosphorescence
spectrum exist at substantially the same position. Accordingly,
both the fluorescence spectrum and the phosphorescence spectrum of
the exciplex can overlap largely with an absorption corresponding
to transition of the guest molecule from the singlet ground state
to the triplet excited state (a broad absorption band of the guest
molecule existing on the longest wavelength side in the absorption
spectrum), and thus a light-emitting element having high energy
transfer efficiency can be obtained.
[0105] In this manner, as a combination of the first
carrier-transport compound 113H.sub.1 and the second
carrier-transport compound 113H.sub.2 in the second light-emitting
layer, the one with which an exciplex is formed is preferable.
Further, when the absorption band of the second phosphorescent
compound 113Db on the lowest energy side and the emission spectrum
of the exciplex overlap with each other, the emission efficiency of
the light-emitting element can be higher. It is preferable that the
difference in equivalent energy value between a peak wavelength in
the absorption band of the second phosphorescent compound 113Db on
the lowest energy side and a peak wavelength of the emission
spectrum of the exciplex be 0.2 eV or less in order that the
spectra largely overlap with each other.
[0106] In the case of the structure illustrated in FIG. 1B, as a
combination of the first host material 113Ha1 and the second host
material 113Ha2, the one with which an exciplex is formed is
preferable. Further, when the absorption band of the first
phosphorescent compound on the lowest energy side and the emission
spectrum of the exciplex overlap with each other, the emission
efficiency of the light-emitting element can be higher. It is
preferable that the difference in equivalent energy value between a
peak wavelength in the absorption band of the first phosphorescent
compound 113Da on the lowest energy side and a peak wavelength of
the emission spectrum of the exciplex be 0.2 eV or less in order
that the spectra largely overlap with each other.
[0107] Light emission from the exciplex is, as described above,
derived from the energy difference between the higher HOMO level of
one of the two kinds of materials forming the exciplex and the
lower LUMO level of the other material. For such a reason, with the
use of the exciplex as a host, a change in the combination of
materials can change the emission spectrum; thus, the emission
spectrum can be easily adjusted to overlap with the absorption of
the phosphorescent compound on the long wavelength side.
[0108] Further, the first host material 113Ha1 and the second host
material 113Ha2 preferably have higher triplet excitation energy
than the first phosphorescent compound 113Da in order that light
emission from the first phosphorescent compound 113Da is not
quenched. Furthermore, the first carrier-transport compound
113H.sub.1 and the second carrier-transport compound 113H.sub.2
preferably have higher triplet excitation energy than the second
phosphorescent compound 113Db in order that light emission from the
second phosphorescent compound 113Db is not quenched.
[0109] A light-emitting element having the above structure can have
high emission efficiency. Further, light emission from
phosphorescent compounds in the light-emitting element can be
provided in a balanced manner.
[0110] The following compounds are examples of an
electron-transport compound and a hole-transport compound that can
be used for the first host material 113Ha1, the second host
material 113Ha2, the first carrier-transport compound 113H.sub.1,
and the second carrier-transport compound 113H.sub.2. Note that one
of the first host material 113Ha1 and the second host material
113Ha2 is an electron-transport compound, and the other of them is
a hole-transport compound. Similarly, one of the first
carrier-transport compound 113H.sub.1 and the second
carrier-transport compound 113H.sub.2 is an electron-transport
compound, and the other of them is a hole-transport compound. As
combinations thereof, the ones with which exciplexes are formed are
preferable.
[0111] The following are examples of the electron-transport
compound: a metal complex such as
bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation:
BeBq.sub.2),
bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)
(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation:
Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation:
ZnPBO), or bis[2-(2-benzothiazolyl)phenolato]zinc(II)
(abbreviation: ZnBTZ); a heterocyclic compound having a polyazole
skeleton such as
2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(abbreviation: PBD),
3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
(abbreviation: TAZ),
1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene
(abbreviation: OXD-7),
9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole
(abbreviation: CO11),
2,2',2''-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)
(abbreviation: TPBI), or
2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole
(abbreviation: mDBTBIm-II); a heterocyclic compound having a
diazine skeleton such as
2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2mDBTPDBq-II),
2-[3'-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline
(abbreviation: 2mDBTBPDBq-II),
2-[3'-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline
(abbreviation: 2mCzBPDBq),
4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation:
4,6mPnP2Pm), or 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine
(abbreviation: 4,6mDBTP2Pm-II); and a heterocyclic compound having
a pyridine skeleton such as
3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:
35DCzPPy) or 1,3,5-tri[(3-pyridyl)phen-3-yl]benzene (abbreviation:
TmPyPB). In particular, a .pi.-electron deficient heteroaromatic
compound is preferable. Among the above materials, a heterocyclic
compound having a diazine skeleton and a heterocyclic compound
having a pyridine skeleton have high reliability and are thus
preferable. Specifically, a heterocyclic compound having a diazine
(pyrimidine or pyrazine) skeleton has a high electron-transport
property to contribute to a reduction in drive voltage.
[0112] The following are examples of the hole-transport compound: a
compound having an aromatic amine skeleton such as
4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),
N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine
(abbreviation: TPD),
4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl
(abbreviation: BSPB),
4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:
BPAFLP), 4-phenyl-3'-(9-phenylfluoren-9-yl)triphenylamine
(abbreviation: mBPAFLP),
4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBA1BP),
4,4'-diphenyl-4''-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBBi1BP),
4-(1-naphthyl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBANB),
4,4'-di(1-naphthyl)-4''-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBNBB),
9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-am-
ine (abbreviation: PCBAF), or
N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9'-bifluoren-2-am-
ine (abbreviation: PCBASF); a compound having a carbazole skeleton
such as 1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),
4,4'-di(N-carbazolyl)biphenyl (abbreviation: CBP),
3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP),
or 3,3'-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound
having a thiophene skeleton such as
4,4',4''-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:
DBT3P-II),
2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene
(abbreviation: DBTFLP-III), or
4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene
(abbreviation: DBTFLP-IV); and a compound having a furan skeleton
such as 4,4',4''-(benzene-1,3,5-triyl)tri(dibenzofuran)
(abbreviation: DBF3P-II) or
4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran
(abbreviation: mmDBFFLBi-II). In particular, a .pi.-electron rich
heteroaromatic compound is preferable. Among the above materials, a
compound having an aromatic amine skeleton and a compound having a
carbazole skeleton are preferable because these compounds are
highly reliable and have high hole-transport properties to
contribute to a reduction in drive voltage.
[0113] A light-emitting element having the above structure has high
emission efficiency. Further, light emission from a plurality of
light-emitting substances in the light-emitting element can be
obtained. The light-emitting element does not have a tandem
structure, and thus its manufacturing process is not complicated
and the amount of power loss due to an intermediate layer is small.
In addition, the light-emitting element has a high utility value as
a white light-emitting element.
[0114] As illustrated in FIGS. 1C and 1D, the light-emitting layer
113 may have a three-layer structure of the first light-emitting
layer 113a, the second light-emitting layer 113b, and a third
light-emitting layer 113c. In this case, the relation between the
first light-emitting layer 113a and the second light-emitting layer
113b is as described above.
[0115] The third light-emitting layer 113c includes a third
phosphorescent compound 113Dc, a third carrier-transport compound
113H.sub.3, and a fourth carrier-transport compound 113H.sub.4. The
emission wavelength of the third phosphorescent compound 113Dc is
longer than that of the second phosphorescent compound 113Db. One
of the third carrier-transport compound 113H.sub.3 and the fourth
carrier-transport compound 113H.sub.4 is an electron-transport
compound, and the other of them is a hole-transport compound. As
examples of compounds that can be used as the third
carrier-transport compound 113H.sub.3 and the fourth
carrier-transport compound 113H.sub.4, the above compounds that can
be used as the first host material 113Ha1, the second host material
113Ha2, the first carrier-transport compound 113H.sub.1, and the
second carrier-transport compound 113H.sub.2 can be given.
[0116] Similarly to the first carrier-transport compound 113H.sub.1
and the second carrier-transport compound 113H.sub.2, or the first
host material 113Ha1 and the second host material 113Ha2, the third
carrier-transport compound 113H.sub.3 and the fourth
carrier-transport compound 113H.sub.4 preferably form an exciplex.
Further, the emission spectrum of the exciplex and the absorption
band of the third phosphorescent compound 113Dc on the longest
wavelength side preferably overlap with each other in order that
energy transfer from the exciplex to the third phosphorescent
compound 113Dc is optimized. It is preferable that the difference
in equivalent energy value between a peak wavelength in the
absorption band of the third phosphorescent compound 113Dc on the
lowest energy side and a peak wavelength of the emission spectrum
of the exciplex be 0.2 eV or less in order that the spectra largely
overlap with each other. Further, the third carrier-transport
compound 113H.sub.3 and the fourth carrier-transport compound
113H.sub.4 preferably have higher triplet excitation energy than
the third phosphorescent compound 113Dc in order that light
emission from the third phosphorescent compound 113Dc is not
quenched.
[0117] The third light-emitting layer 113c emits light in such a
manner that recombination energy generated in the recombination
region in the first light-emitting layer 113a or in the vicinity of
the interface between the first light-emitting layer 113a and the
second light-emitting layer 113b is transferred to the third
light-emitting layer 113c through the second light-emitting layer
113b. Therefore, the triplet excitation energies of the first
carrier-transport compound 113H.sub.1 and the second
carrier-transport compound 113H.sub.2 are preferably higher than
the triplet excitation energy of one of the third carrier-transport
compound 113H.sub.3 and the fourth carrier-transport compound
113H.sub.4.
[0118] The third light-emitting layer 113c preferably has the same
carrier-transport property as the second light-emitting layer 113b
in order that a recombination region is in the first light-emitting
layer 113a or in the vicinity of the interface between the first
light-emitting layer 113a and the second light-emitting layer 113b.
Further, one of or both the electron-transport compound and the
hole-transport compound in the third carrier-transport compound
113H.sub.3 and the fourth carrier-transport compound 113H.sub.4 may
be the same as one of or both the electron-transport compound and
the hole-transport compound in the first carrier-transport compound
113H.sub.1 and the second carrier-transport compound 113H.sub.2. In
this case, the materials are used in common between different
layers, which is advantageous in cost.
[0119] In the light-emitting element in FIG. 1E including the
light-emitting layer 113 illustrated in FIG. 1C or 1D, the first
light-emitting layer 113a may be formed on the anode side or
cathode side.
[0120] In the case where the first light-emitting layer 113a is
formed on the anode side, the first light-emitting layer 113a is
preferably a layer having a hole-transport property and the second
light-emitting layer 113b and the third light-emitting layer 113c
are preferably layers each having an electron-transport property.
In the first light-emitting layer, the first host material 113Ha1
may have a hole-transport property in the case of the structure
illustrated in FIG. 1C. The carrier-transport property of the first
light-emitting layer 113a can be adjusted by changing the mixture
ratio of the first host material 113Ha1 to the second host material
113Ha2 (that is, the electron-transport compound to the
hole-transport compound) in the case of the structure illustrated
in FIG. 1D. In a similar manner, the second light-emitting layer
113b and the third light-emitting layer 113c can have desired
carrier-transport properties by changing the mixture ratio of the
first carrier-transport compound 113H.sub.1 to the second
carrier-transport compound 113H.sub.2, the third carrier-transport
compound 113H.sub.3, and the fourth carrier-transport compound
113H.sub.4.
[0121] In the case where the first light-emitting layer 113a is
formed on the cathode side, the first light-emitting layer 113a is
preferably a layer having an electron-transport property and the
second light-emitting layer 113b and the third light-emitting layer
113c are preferably layers each having a hole-transport property.
In the first light-emitting layer 113a, the first host material
113Ha1 may have an electron-transport property in the case of the
structure illustrated in FIG. 1C. The carrier-transport property of
the first light-emitting layer 113a can be adjusted by changing the
mixture ratio of the first host material 113Ha1 to the second host
material 113Ha2 (that is, the electron-transport compound to the
hole-transport compound) in the case of the structure illustrated
in FIG. 1D. In a similar manner, the second light-emitting layer
113b and the third light-emitting layer 113c can have desired
carrier-transport properties by changing the mixture ratio of the
first carrier-transport compound 113H.sub.1 to the second
carrier-transport compound 113H.sub.2, the third carrier-transport
compound 113H.sub.3, and the fourth carrier-transport compound
113H.sub.4.
[0122] Energy is transferred through the second light-emitting
layer 113b, and thus is not transferred to the third light-emitting
layer 113c when the thickness of the second light-emitting layer
113b is too large; in this case, light emission from the third
phosphorescent compound 113Dc cannot be obtained. Therefore, in
order that light emission from the third light-emitting layer 113c
is obtained, the thickness of the second light-emitting layer 113b
is preferably greater than or equal to 2 nm and less than or equal
to 20 nm, more preferably greater than or equal to 5 nm and less
than or equal to 10 nm.
[0123] In the light-emitting element of this embodiment including
the light-emitting layer 113 illustrated in FIG. 1C or 1D, a
compound exhibiting blue light emission, a compound exhibiting
green light emission, and a compound exhibiting red light emission
are used as the first phosphorescent compound 113Da, the second
phosphorescent compound 113Db, and the third phosphorescent
compound 113Dc, respectively; thus, favorable white light emission
(e.g., white light emission that meets the standards defined by
Japanese Industrial Standards (JIS)) can be obtained. Such white
light emission has an excellent color rendering property. Such a
white light-emitting element is significantly suitable for
lighting.
[0124] A light-emitting element having the above structure includes
a plurality of light-emitting substances and has high emission
efficiency. Further, light emission from the plurality of
light-emitting substances in the light-emitting element can be
provided in a balanced manner.
Embodiment 2
[0125] In this embodiment, a detailed example of the structure of
the light-emitting element described in Embodiment 1 will be
described below with reference to FIG. 1E.
[0126] A light-emitting element in this embodiment includes,
between a pair of electrodes, an EL layer including a plurality of
layers. In this embodiment, the light-emitting element includes the
first electrode 101, the second electrode 102, and the EL layer 103
which is provided between the first electrode 101 and the second
electrode 102. The following description in this embodiment is made
on the assumption that the first electrode 101 functions as an
anode and the second electrode 102 functions as a cathode. In other
words, when a voltage is applied between the first electrode 101
and the second electrode 102 so that the potential of the first
electrode 101 is higher than that of the second electrode 102,
light emission can be obtained.
[0127] Since the first electrode 101 functions as the anode, the
first electrode 101 is preferably formed using any of metals,
alloys, conductive compounds with a high work function
(specifically, a work function of 4.0 eV or more), mixtures
thereof, and the like. Specifically, for example, indium oxide-tin
oxide (ITO: indium tin oxide), indium oxide-tin oxide containing
silicon or silicon oxide, indium oxide-zinc oxide, indium oxide
containing tungsten oxide and zinc oxide (IWZO), and the like can
be given. Films of these conductive metal oxides are usually formed
by a sputtering method but may be formed by application of a
sol-gel method or the like. In an example of the formation method,
indium oxide-zinc oxide is deposited by a sputtering method using a
target obtained by adding 1 wt % to 20 wt % of zinc oxide to indium
oxide. Further, a film of indium oxide containing tungsten oxide
and zinc oxide (IWZO) can be formed by a sputtering method using a
target in which tungsten oxide and zinc oxide are added to indium
oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %, respectively.
Besides, gold (Au), platinum (Pt), nickel (Ni), tungsten (W),
chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper
(Cu), palladium (Pd), nitrides of metal materials (e.g., titanium
nitride), and the like can be given. Graphene can also be used.
Note that when a composite material described later is used for a
layer which is in contact with the first electrode 101 in the EL
layer 103, an electrode material can be selected regardless of its
work function.
[0128] There is no particular limitation on the stacked-layer
structure of the EL layer 103 as long as the light-emitting layer
113 has the structure described in Embodiment 1. For example, the
EL layer 103 can be formed by combining a hole-injection layer, a
hole-transport layer, the light-emitting layer, an
electron-transport layer, an electron-injection layer, a
carrier-blocking layer, and the like as appropriate. In this
embodiment, the EL layer 103 has a structure in which the
hole-injection layer 111, the hole-transport layer 112, the
light-emitting layer 113, the electron-transport layer 114, and the
electron-injection layer 115 are stacked in this order over the
first electrode 101. Specific examples of materials used for each
layer are given below.
[0129] The hole-injection layer 111 is a layer containing a
substance having a high hole-injection property. Molybdenum oxide,
vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide,
or the like can be used. Alternatively, the hole-injection layer
111 can be formed using a phthalocyanine-based compound such as
phthalocyanine (abbreviation: H.sub.2Pc) or copper phthalocyanine
(abbreviation: CuPc), an aromatic amine compound such as
4,4'-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl
(abbreviation: DPAB) or
N,N'-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N'-diphenyl-(1,1'-
-biphenyl)-4,4'-diamine (abbreviation: DNTPD), a high molecular
compound such as poly(ethylenedioxythiophene)/poly(styrenesulfonic
acid) (abbreviation: PEDOT/PSS), or the like.
[0130] Alternatively, a composite material in which a substance
having a hole-transport property contains a substance having an
acceptor property can be used for the hole-injection layer 111.
Note that the use of such a substance having a hole-transport
property which contains a substance having an acceptor property
enables selection of a material used to form an electrode
regardless of its work function. In other words, besides a material
having a high work function, a material having a low work function
can also be used for the first electrode 101. As the substance
having an acceptor property,
7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:
F.sub.4-TCNQ), chloranil, and the like can be given. In addition,
transition metal oxides can be given. Oxides of the metals that
belong to Groups 4 to 8 of the periodic table can be given.
Specifically, vanadium oxide, niobium oxide, tantalum oxide,
chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide,
and rhenium oxide are preferable in that their electron-accepting
property is high. Among these oxides, molybdenum oxide is
particularly preferable in that it is stable in the air, has a low
hygroscopic property, and is easy to handle.
[0131] As the substance having a hole-transport property which is
used for the composite material, any of a variety of organic
compounds such as aromatic amine compounds, carbazole derivatives,
aromatic hydrocarbons, and high molecular compounds (e.g.,
oligomers, dendrimers, or polymers) can be used. Note that the
organic compound used for the composite material is preferably an
organic compound having a high hole-transport property.
Specifically, a substance having a hole mobility of 10.sup.-6
cm.sup.2/Vs or more is preferably used. Organic compounds that can
be used as the substance having a hole-transport property in the
composite material are specifically given below.
[0132] Examples of the aromatic amine compounds are
N,N'-di(p-tolyl)-N,N'-diphenyl-p-phenylenediamine (abbreviation:
DTDPPA), 4,4'-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl
(abbreviation: DPAB),
N,N'-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N'-diphenyl-(1,1'-b-
iphenyl)-4,4'-diamine (abbreviation: DNTPD),
1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene
(abbreviation: DPA3B), and the like.
[0133] Specific examples of the carbazole derivatives that can be
used for the composite material are
3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole
(abbreviation: PCzPCA1),
3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole
(abbreviation: PCzPCA2),
3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole
(abbreviation: PCzPCN1), and the like.
[0134] Other examples of the carbazole derivatives that can be used
for the composite material are 4,4'-di(N-carbazolyl)biphenyl
(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene
(abbreviation: TCPB),
9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:
CzPA), 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene,
and the like.
[0135] Examples of the aromatic hydrocarbons that can be used for
the composite material are
2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),
2-tert-butyl-9,10-di(1-naphthyl)anthracene,
9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),
2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation:
t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),
9,10-diphenylanthracene (abbreviation: DPAnth),
2-tert-butylanthracene (abbreviation: t-BuAnth),
9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA),
2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,
9,10-bis[2-(1-naphthyl)phenyl]anthracene,
2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,
2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9'-bianthryl,
10,10'-diphenyl-9,9'-bianthryl,
10,10'-bis(2-phenylphenyl)-9,9'-bianthryl,
10,10'-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9'-bianthryl,
anthracene, tetracene, rubrene, perylene,
2,5,8,11-tetra(tert-butyl)perylene, and the like. Besides,
pentacene, coronene, or the like can also be used. The aromatic
hydrocarbon which has a hole mobility of 1.times.10.sup.-6
cm.sup.2/Vs or more and which has 14 to 42 carbon atoms is
particularly preferable.
[0136] Note that the aromatic hydrocarbons that can be used for the
composite material may have a vinyl skeleton. Examples of the
aromatic hydrocarbon having a vinyl group are
4,4'-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi),
9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation:
DPVPA), and the like.
[0137] A polymeric compound such as poly(N-vinylcarbazole)
(abbreviation: PVK) poly(4-vinyltriphenylamine) (abbreviation:
PVTPA),
poly[N-(4-{N'-[4-(4-diphenylamino)phenyl]phenyl-N'-phenylamino}phenyl)met-
hacrylamide] (abbreviation: PTPDMA), or
poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine]
(abbreviation: poly-TPD) can also be used.
[0138] By providing a hole-injection layer, a high hole-injection
property can be achieved to allow a light-emitting element to be
driven at a low voltage.
[0139] The hole-transport layer 112 is a layer that contains a
substance having a hole-transport property. Examples of the
substance having a hole-transport property are aromatic amine
compounds such as 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl
(abbreviation: NPB),
N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine
(abbreviation: TPD),
4,4',4''-tris(N,N'-diphenylamino)triphenylamine (abbreviation:
TDATA),
4,4',4''-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine
(abbreviation: MTDATA),
4,4'-bis[N-(Spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl
(abbreviation: BSPB),
4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:
BPAFLP), and the like. The substances mentioned here have high
hole-transport properties and are mainly ones that have a hole
mobility of 10.sup.-6 cm.sup.2/Vs or more. An organic compound
given as an example of the substance having a hole-transport
property in the composite material described above can also be used
for the hole-transport layer 112. A polymeric compound such as
poly(N-vinylcarbazole) (abbreviation: PVK) or
poly(4-vinyltriphenylamine) (abbreviation: PVTPA) can also be used.
Note that the layer that contains a substance having a
hole-transport property is not limited to a single layer, and may
be a stack of two or more layers including any of the above
substances.
[0140] The light-emitting layer 113 has the structure described in
Embodiment 1. Therefore, the light-emitting element of this
embodiment has high emission efficiency, and light emission from a
plurality of phosphorescent compounds in the light-emitting element
can be provided in a balanced manner. Embodiment 1 is to be
referred to for the main structure of the light-emitting layer
113.
[0141] There is no particular limitation on materials that can be
used as the first to third phosphorescent compounds 113Da to 113Dc
in the light-emitting layer 113 as long as they have the relation
described in Embodiment 1. The following can be given as examples
of the first to third phosphorescent compounds 113Da to 113Dc.
[0142] The following are the specific examples: an organometallic
iridium complex having a 4H-triazole skeleton, such as
tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-.-
kappa.N2]phenyl-.kappa.C}iridium(III) (abbreviation:
Ir(mpptz-dmp).sub.3),
tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)
(abbreviation: Ir(Mptz).sub.3), or
tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)
(abbreviation: Ir(iPrptz-3b).sub.3); an organometallic iridium
complex having a 1H-triazole skeleton, such as
tris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III-
) (abbreviation: Ir(Mptz1-mp).sub.3) or
tris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)
(abbreviation: Ir(Prptz1-Me).sub.3); an organometallic iridium
complex having an imidazole skeleton, such as
fac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)
(abbreviation: Ir(iPrpmi).sub.3), or
tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridiu-
m(III) (abbreviation: Ir(dmpimpt-Me).sub.3); and an organometallic
iridium complex in which a phenylpyridine derivative having an
electron-withdrawing group is a ligand, such as
bis[2-(4',6'-difluorophenyl)pyridinato-N,
C.sup.2']iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation:
FIr6),
bis[2-(4',6'-difluorophenyl)pyridinato-N,C.sup.2']iridium(III)
picolinate (abbreviation: Flrpic),
bis[2-(3',5'-bistrifluoromethylphenyl)pyridinato-N,C.sup.2']iridium(III)p-
icolinate (abbreviation: Ir(CF.sub.3ppy).sub.2(pic)), or
bis[2-(4',6'-difluorophenyl)pyridinato-N,C.sup.2']iridium(III)
acetylacetonate (abbreviation: FIracac). These are compounds
emitting blue phosphorescence and have an emission peak at 440 nm
to 520 nm. Among the above compounds, an organometallic iridium
complex having a polyazole skeleton such as a 4H-triazole skeleton,
a 1H-triazole skeleton, or an imidazole skeleton has a high
hole-trapping property. Therefore, it is preferable that any of
these compounds be used as the first phosphorescent compound in the
light-emitting element of one embodiment of the present invention,
the first light-emitting layer be provided closer to the cathode
than the second light-emitting layer, and the second light-emitting
layer have a hole-transport property (specifically, the second host
material be a hole-transport material), in which case a
recombination region of carriers can be easily controlled to be in
the first light-emitting layer. Note that an organometallic iridium
complex having a 4H-triazole skeleton has excellent reliability and
emission efficiency and thus is especially preferable.
[0143] The following are the specific examples: an organometallic
iridium complex having a pyrimidine skeleton, such as
tris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:
Ir(mppm).sub.3), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)
(abbreviation: Ir(tBuppm).sub.3),
(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)
(abbreviation: Ir(mppm).sub.2(acac)),
(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)
(abbreviation: Ir(tBuppm).sub.2(acac)),
(acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium(III)
(abbreviation: Ir(nbppm).sub.2(acac)),
(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iri-
dium(III) (abbreviation: Ir(mpmppm).sub.2(acac)), or
(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)
(abbreviation: Ir(dppm).sub.2(acac)); an organometallic iridium
complex having a pyrazine skeleton, such as
(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)
(abbreviation: Ir(mppr-Me).sub.2(acac)) or
(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)
(abbreviation: Ir(mppr-iPr).sub.2(acac)); an organometallic iridium
complex having a pyridine skeleton, such as
tris(2-phenylpyridinato-N,C.sup.2')iridium(III) (abbreviation:
Ir(ppy).sub.3),
bis(2-phenylpyridinato-N,C.sup.2')iridium(III)acetylacetonate
(abbreviation: Ir(ppy).sub.2acac),
bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:
Ir(bzq).sub.2(acac)), tris(benzo[h]quinolinato)iridium(III)
(abbreviation: Ir(bzq).sub.3),
tris(2-phenylquinolinato-N,C.sup.2')iridium(III) (abbreviation:
Ir(pq).sub.3), or
bis(2-phenylquinolinato-N,C.sup.2')iridium(III)acetylacetonate
(abbreviation: Ir(pq).sub.2(acac)); and a rare earth metal complex
such as tris(acetylacetonato)(monophenanthroline)terbium(III)
(abbreviation: Tb(acac).sub.3(Phen)). These are mainly compounds
emitting green phosphorescence and have an emission peak at 500 nm
to 600 nm. Among the above compounds, an organometallic iridium
complex having a diazine skeleton such as a pyrimidine skeleton or
a pyrazine skeleton has a low hole-trapping property and a high
electron-trapping property. Therefore, it is preferable that any of
these compounds be used as the first phosphorescent compound in the
light-emitting element of one embodiment of the present invention,
the first light-emitting layer be provided closer to the anode than
the second light-emitting layer, and the second light-emitting
layer have an electron-transport property (specifically, the second
host material be an electron-transport material), in which case a
recombination region of carriers can be easily controlled to be in
the first light-emitting layer. Note that an organometallic iridium
complex having a pyrimidine skeleton has distinctively high
reliability and emission efficiency and thus is especially
preferable.
[0144] Still other examples are an organometallic iridium complex
having a pyrimidine skeleton such as
(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(II-
I) (abbreviation: Ir(5mdppm).sub.2(dibm)),
bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)
(abbreviation: Ir(5mdppm).sub.2(dpm)), or
bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III)
(abbreviation: Ir(d1npm).sub.2(dpm)); an organometallic iridium
complex having a pyrazine skeleton such as
(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)
(abbreviation: Ir(tppr).sub.2(acac)),
bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)
(abbreviation: Ir(tppr).sub.2(dpm)), or
(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)
(abbreviation: Ir(Fdpq).sub.2(acac)); an organometallic iridium
complex having a pyridine skeleton such as
tris(1-phenylisoquinolinato-N,C.sup.2')iridium(III) (abbreviation:
Ir(piq).sub.3) or
bis(1-phenylisoquinolinato-N,C.sup.2')iridium(III) acetylacetonate
(abbreviation: Ir(piq).sub.2(acac)); a platinum complex such as
2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)
(abbreviation: PtOEP); and a rare earth metal complex such as
tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)
(abbreviation: Eu(DBM).sub.3(Phen)) or
tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(-
III) (abbreviation: Eu(TTA).sub.3(Phen)). These are compounds
emitting red phosphorescence and have an emission peak at 600 nm to
700 nm. Among the above compounds, an organometallic iridium
complex having a diazine skeleton such as a pyrimidine skeleton or
a pyrazine skeleton has a low hole-trapping property and a high
electron-trapping property. Therefore, it is preferable that an
organometallic iridium complex having a diazine skeleton be used as
the second phosphorescent compound, the first light-emitting layer
be provided closer to the cathode than the second light-emitting
layer, and the second light-emitting layer have a hole-transport
property (specifically, the second host material be a
hole-transport material), in which case a recombination region of
carriers can be easily controlled to be in the first light-emitting
layer. Note that an organometallic iridium complex having a
pyrimidine skeleton has distinctively high reliability and emission
efficiency and thus is especially preferable. Further, because an
organometallic iridium complex having a pyrazine skeleton can
provide red light emission with favorable chromaticity, the use of
the organometallic iridium complex in a white light-emitting
element improves a color rendering property of the white
light-emitting element.
[0145] It is also possible to select the first to third
phosphorescent compounds 113Da to 113Dc from known phosphorescent
materials in addition to the above phosphorescent compounds.
[0146] Note that the phosphorescent compounds (the first to third
phosphorescent compounds 113Da to 113Dc) may be replaced with a
material exhibiting thermally activated delayed fluorescence, i.e.,
a thermally activated delayed fluorescence (TADF) material. Here,
the term "delayed fluorescence" refers to light emission having a
spectrum similar to normal fluorescence and an extremely long
lifetime. The lifetime is 10.sup.-6 seconds or longer, preferably
10.sup.-3 seconds or longer. Specific examples of the thermally
activated delayed fluorescence material include a fullerene, a
derivative thereof, an acridine derivative such as proflavine, and
eosin. Other examples include a metal-containing porphyrin, such as
a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin
(Sn), platinum (Pt), indium (In), or palladium (Pd). Examples of
the metal-containing porphyrin include a protoporphyrin-tin
fluoride complex (SnF.sub.2(Proto IX)), a mesoporphyrin-tin
fluoride complex (SnF.sub.2(Meso IX)), a hematoporphyrin-tin
fluoride complex (SnF.sub.2(Hemato IX)), a coproporphyrin
tetramethyl ester-tin fluoride complex (SnF.sub.2(Copro III-4Me)),
an octaethylporphyrin-tin fluoride complex (SnF.sub.2(OEP)), an
etioporphyrin-tin fluoride complex (SnF.sub.2(Etio I)), and an
octaethylporphyrin-platinum chloride complex (PtCl.sub.2OEP).
Alternatively, a heterocyclic compound including a .pi.-electron
rich heteroaromatic ring and a .pi.-electron deficient
heteroaromatic ring can be used, such as
2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-tri-
azine (PIC-TRZ). Note that a material in which the .pi.-electron
rich heteroaromatic ring is directly bonded to the .pi.-electron
deficient heteroaromatic ring is particularly preferably used
because both the donor property of the .pi.-electron rich
heteroaromatic ring and the acceptor property of the .pi.-electron
deficient heteroaromatic ring are increased and the energy
difference between the S.sub.1 level and the T.sub.1 level becomes
small.
[0147] Materials that can be used for the first host material
113Ha1, the second host material 113Ha2, the first
carrier-transport compound 113H.sub.1, the second carrier-transport
compound 113H.sub.2, the third carrier-transport compound
113H.sub.3, and the fourth carrier-transport compound 113H.sub.4
are described in Embodiment 1; thus, the description thereof is not
given here.
[0148] For formation of the light-emitting layer 113,
co-evaporation by a vacuum evaporation method can be used, or
alternatively an inkjet method, a spin coating method, a dip
coating method, or the like using a mixed solution can be used.
[0149] The electron-transport layer 114 is a layer containing a
substance having an electron-transport property. For example, a
layer containing a metal complex having a quinoline skeleton or a
benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum
(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum
(abbreviation: Almq.sub.3),
bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation:
BeBq.sub.2), or
bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum
(abbreviation: BAlq), or the like can be used. Alternatively, a
metal complex having an oxazole-based or thiazole-based ligand,
such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation:
Zn(BOX).sub.2) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc
(abbreviation: Zn(BTZ).sub.2), or the like can be used. Besides the
metal complexes,
2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(abbreviation: PBD),
1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene
(abbreviation: OXD-7),
3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),
bathocuproine (abbreviation: BCP), or the like can also be used.
The substances mentioned here have high electron-transport
properties and are mainly ones that have an electron mobility of
10.sup.-6 cm.sup.2/Vs or more. Note that any of the above-described
host materials having electron-transport properties may be used for
the electron-transport layer 114.
[0150] The electron-transport layer 114 is not limited to a single
layer, and may be a stack of two or more layers containing any of
the above substances.
[0151] Between the electron-transport layer and the light-emitting
layer, a layer that controls transport of electron carriers may be
provided. This is a layer formed by addition of a small amount of a
substance having a high electron-trapping property to the
aforementioned material having a high electron-transport property,
and the layer is capable of adjusting carrier balance by retarding
transport of electron carriers. Such a structure is very effective
in preventing a problem (such as a reduction in element lifetime)
caused when electrons pass through the light-emitting layer.
[0152] In addition, the electron-injection layer 115 may be
provided in contact with the second electrode 102 between the
electron-transport layer 114 and the second electrode 102. For the
electron-injection layer 115, an alkali metal, an alkaline earth
metal, or a compound thereof, such as lithium fluoride (LW), cesium
fluoride (CsF), or calcium fluoride (CaF.sub.2), can be used. For
example, a layer that is formed using a substance having an
electron-transport property and contains an alkali metal, an
alkaline earth metal, or a compound thereof can be used. Note that
a layer that is formed using a substance having an
electron-transport property and contains an alkali metal or an
alkaline earth metal is preferably used as the electron-injection
layer 115, in which case electron injection from the second
electrode 102 is efficiently performed.
[0153] For the second electrode 102, any of metals, alloys,
electrically conductive compounds, and mixtures thereof which have
a low work function (specifically, a work function of 3.8 eV or
less) or the like can be used. Specific examples of such a cathode
material are elements belonging to Groups 1 and 2 of the periodic
table, such as alkali metals (e.g., lithium (Li) and cesium (Cs)),
magnesium (Mg), calcium (Ca), and strontium (Sr), alloys thereof
(e.g., MgAg and AlLi), rare earth metals such as europium (Eu) and
ytterbium (Yb), alloys thereof, and the like. However, when the
electron-injection layer is provided between the second electrode
102 and the electron-transport layer, for the second electrode 102,
any of a variety of conductive materials such as Al, Ag, indium
oxide-tin oxide, or indium oxide-tin oxide containing silicon or
silicon oxide can be used regardless of the work function. Films of
these conductive materials can be formed by a sputtering method, an
inkjet method, a spin coating method, or the like.
[0154] Any of a variety of methods can be used to form the EL layer
103 regardless whether it is a dry process or a wet process. For
example, a vacuum evaporation method, an inkjet method, a spin
coating method, or the like may be used. Different formation
methods may be used for the electrodes or the layers.
[0155] In addition, the electrode may be formed by a wet method
using a sol-gel method, or by a wet method using paste of a metal
material. Alternatively, the electrode may be formed by a dry
method such as a sputtering method or a vacuum evaporation
method.
[0156] In the light-emitting element having the above-described
structure, current flows due to a potential difference between the
first electrode 101 and the second electrode 102, and holes and
electrons recombine in the light-emitting layer 113 which contains
a substance having a high light-emitting property, so that light is
emitted. That is, a light-emitting region is formed in the
light-emitting layer 113.
[0157] Light emission is extracted out through one of or both the
first electrode 101 and the second electrode 102. Therefore, one of
or both the first electrode 101 and the second electrode 102 are
light-transmitting electrodes. In the case where only the first
electrode 101 is a light-transmitting electrode, light emission is
extracted through the first electrode 101. In the case where only
the second electrode 102 is a light-transmitting electrode, light
emission is extracted through the second electrode 102. In the case
where both the first electrode 101 and the second electrode 102 are
light-transmitting electrodes, light emission is extracted through
the first electrode 101 and the second electrode 102.
[0158] The structure of the layers provided between the first
electrode 101 and the second electrode 102 is not limited to the
above-described structure. Preferably, a light-emitting region
where holes and electrons recombine is positioned away from the
first electrode 101 and the second electrode 102 so that quenching
due to the proximity of the light-emitting region and a metal used
for electrodes and carrier-injection layers can be prevented.
[0159] Further, in order that transfer of energy from an exciton
generated in the light-emitting layer can be suppressed,
preferably, the hole-transport layer and the electron-transport
layer which are in contact with the light-emitting layer 113,
particularly a carrier-transport layer in contact with a side
closer to the light-emitting region in the light-emitting layer
113, are formed using a substance having higher triplet excitation
energy than the substance in the light-emitting layer.
[0160] A light-emitting element in this embodiment is preferably
fabricated over a substrate of glass, plastic, or the like. As the
way of stacking layers over the substrate, layers may be
sequentially stacked from the first electrode 101 side or
sequentially stacked from the second electrode 102 side. In a
light-emitting device, although one light-emitting element may be
formed over one substrate, a plurality of light-emitting elements
may be formed over one substrate. With a plurality of
light-emitting elements as described above formed over one
substrate, a lighting device in which elements are separated or a
passive matrix light-emitting device can be manufactured. A
light-emitting element may be formed over an electrode electrically
connected to a thin film transistor (TFT), for example, which is
formed over a substrate of glass, plastic, or the like, so that an
active matrix light-emitting device in which the TET controls the
drive of the light-emitting element can be manufactured. Note that
there is no particular limitation on the structure of the TFT,
which may be a staggered TFT or an inverted staggered TFT. In
addition, crystallinity of a semiconductor used for the TFT is not
particularly limited either; an amorphous semiconductor or a
crystalline semiconductor may be used. In addition, a driver
circuit formed in a TFT substrate may be formed with an n-type TFT
and a p-type TFT, or with either an n-type TFT or a p-type TFT.
[0161] Note that this embodiment can be combined with any of the
other embodiments as appropriate.
Embodiment 3
[0162] In this embodiment, an embodiment of a light-emitting
element with a structure in which a plurality of light-emitting
units are stacked (hereinafter, also referred to as "stacked-type
element") will be described with reference to FIG. 2. This
light-emitting element is a light-emitting element including a
plurality of light-emitting units between a first electrode and a
second electrode. One light-emitting unit has the same structure as
the EL layer 103 illustrated in FIG. 1E. In other words, the
light-emitting element illustrated in FIG. 1E includes one
light-emitting unit while the light-emitting element in this
embodiment includes a plurality of light-emitting units.
[0163] In FIG. 2, a first light-emitting unit 511 and a second
light-emitting unit 512 are stacked between a first electrode 501
and a second electrode 502, and a charge-generation layer 513 is
provided between the first light-emitting unit 511 and the second
light-emitting unit 512. The first electrode 501 and the second
electrode 502 correspond, respectively, to the first electrode 101
and the second electrode 102 in FIG. 1E, and the materials given in
the description with reference to FIG. 1E can be used. Further, the
first light-emitting unit 511 and the second light-emitting unit
512 may have the same structure or different structures.
[0164] The charge-generation layer 513 contains a composite
material of an organic compound and a metal oxide. As this
composite material of an organic compound and a metal oxide, the
composite material that can be used for the hole-injection layer
111 illustrated in FIG. 1E can be used. As the organic compound, a
variety of compounds such as an aromatic amine compound, a
carbazole compound, an aromatic hydrocarbon, and a high molecular
compound (such as an oligomer, a dendrimer, or a polymer) can be
used. An organic compound having a hole mobility of
1.times.10.sup.-6 cm.sup.2/Vs or higher is preferably used.
However, any other substance may be used as long as the substance
has a hole-transport property higher than an electron-transport
property. The composite material of an organic compound and a metal
oxide has a high carrier-injection property and a high
carrier-transport property; thus, low-voltage driving and
low-current driving can be achieved. Note that when a surface of a
light-emitting unit on the anode side is in contact with a charge
generation layer, the charge generation layer can also serve as a
hole-transport layer of the light-emitting unit; thus, a
hole-transport layer does not need to be formed in the
light-emitting unit.
[0165] The charge-generation layer 513 may have a stacked-layer
structure of a layer containing the composite material of an
organic compound and a metal oxide and a layer containing another
material. For example, the charge-generation layer 513 may have a
stacked-layer structure of a layer containing the composite
material of an organic compound and a metal oxide and a layer
containing a compound of a substance selected from
electron-donating substances and a compound having a high
electron-transport property. Moreover, the charge-generation layer
513 may have a stacked-layer structure of a layer containing the
composite material of an organic compound and a metal oxide and a
transparent conductive film.
[0166] The charge-generation layer 513 provided between the first
light-emitting unit 511 and the second light-emitting unit 512 may
have any structure as far as electrons can be injected to a
light-emitting unit on one side and holes can be injected to a
light-emitting unit on the other side when a voltage is applied
between the first electrode 501 and the second electrode 502. For
example, in FIG. 2, any layer can be used as the charge generation
layer 513 as far as the layer injects electrons into the first
light-emitting unit 511 and holes into the second light-emitting
unit 512 when a voltage is applied such that the potential of the
first electrode is higher than that of the second electrode.
[0167] Although the light-emitting element having two
light-emitting units is illustrated in FIG. 2, one embodiment of
the present invention can be similarly applied to a light-emitting
element in which three or more light-emitting units are stacked.
With a plurality of light-emitting units partitioned by the
charge-generation layer between a pair of electrodes as in the
light-emitting element of this embodiment, it is possible to
provide a light-emitting element that can emit light with high
luminance with the current density kept low and has a long
lifetime. Moreover, a light-emitting device having low driving
voltage and lower power consumption can be achieved.
[0168] Further, when emission colors of the light-emitting units
are made different, light emission having a desired color can be
obtained from the light-emitting element as a whole. For example,
in the light-emitting element having two light-emitting units, when
the first light-emitting unit emits light of red and green and the
second light-emitting unit emits light of blue, it is possible to
obtain a light-emitting element from which white light is emitted
from the whole light-emitting element.
[0169] When the above-described structure of the light-emitting
layer 113 is applied to at least one of the plurality of units, the
number of manufacturing steps of the unit can be reduced; thus, a
multicolor light-emitting element which is advantageous for
practical application can be provided.
[0170] The above-described structure can be combined with any of
the structures in this embodiment and the other embodiments.
Embodiment 4
[0171] In this embodiment, a light-emitting device manufactured
using the light-emitting element described in any of Embodiments 1
to 3 will be described.
[0172] In this embodiment, a light-emitting device manufactured
using the light-emitting element described in any of Embodiments 1
to 3 will be described with reference to FIGS. 3A and 3B. Note that
FIG. 3A is a top view of the light-emitting device and FIG. 3B is a
cross-sectional view taken along the lines A-B and C-D in FIG. 3A.
This light-emitting device includes a driver circuit portion
(source line driver circuit) 601, a pixel portion 602, and a driver
circuit portion (gate line driver circuit) 603, which are to
control light emission of a light-emitting element and illustrated
with dotted lines. Reference numeral 604 denotes a sealing
substrate; 605, a sealing material; and 607, a space surrounded by
the sealing material 605.
[0173] Reference numeral 608 denotes a wiring for transmitting
signals to be input to the source line driver circuit 601 and the
gate line driver circuit 603 and receiving signals such as a video
signal, a clock signal, a start signal, and a reset signal from an
FPC (flexible printed circuit) 609 serving as an external input
terminal. Although only the FPC is illustrated here, a printed
wiring board (PWB) may be attached to the FPC. The light-emitting
device in the present specification includes, in its category, not
only the light-emitting device itself but also the light-emitting
device provided with the FPC or the PWB.
[0174] Next, a cross-sectional structure is described with
reference to FIG. 3B. The driver circuit portion and the pixel
portion are formed over an element substrate 610; FIG. 3B shows the
source line driver circuit 601, which is a driver circuit portion,
and one of the pixels in the pixel portion 602.
[0175] As the source line driver circuit 601, a CMOS circuit in
which an n-channel TFT 623 and a p-channel TFT 624 are combined is
formed. In addition, the driver circuit may be formed with any of a
variety of circuits such as a CMOS circuit, a PMOS circuit, or an
NMOS circuit. Although a driver integrated type in which the driver
circuit is formed over the substrate is illustrated in this
embodiment, the driver circuit is not necessarily formed over the
substrate, and the driver circuit can be formed outside, not over
the substrate.
[0176] The pixel portion 602 includes a plurality of pixels
including a switching TFT 611, a current controlling TFT 612, and a
first electrode 613 electrically connected to a drain of the
current controlling TFT 612. Note that to cover an end portion of
the first electrode 613, an insulator 614 is formed, for which a
positive photosensitive acrylic resin film is used here.
[0177] In order to improve coverage, the insulator 614 is formed to
have a curved surface with curvature at its upper or lower end
portion. For example, in the case where positive photosensitive
acrylic is used for a material of the insulator 614, only the upper
end portion of the insulator 614 preferably has a curved surface
with a curvature radius (0.2 .mu.m to 3 .mu.m). As the insulator
614, either a negative photosensitive resin or a positive
photosensitive resin can be used.
[0178] An EL layer 616 and a second electrode 617 are formed over
the first electrode 613. Here, as a material used for the first
electrode 613 functioning as an anode, a material having a high
work function is preferably used. For example, a single-layer film
of an ITO film, an indium tin oxide film containing silicon, an
indium oxide film containing zinc oxide at 2 wt % to 20 wt %, a
titanium nitride film, a chromium film, a tungsten film, a Zn film,
a Pt film, or the like, a stack of a titanium nitride film and a
film containing aluminum as its main component, a stack of three
layers of a titanium nitride film, a film containing aluminum as
its main component, and a titanium nitride film, or the like can be
used. The stacked-layer structure enables low wiring resistance,
favorable ohmic contact, and a function as an anode.
[0179] The EL layer 616 is formed by any of a variety of methods
such as an evaporation method using an evaporation mask, an inkjet
method, and a spin coating method. The EL layer 616 has the
structure described in any of Embodiments 1 to 3. Further, for
another material included in the EL layer 616, any of low
molecular-weight compounds and polymeric compounds (including
oligomers and dendrimers) may be used.
[0180] As a material used for the second electrode 617, which is
formed over the EL layer 616 and functions as a cathode, a material
having a low work function (e.g., Al, Mg, Li, Ca, or an alloy or a
compound thereof, such as MgAg, MgIn, or Al--Li) is preferably
used. In the case where light generated in the EL layer 616 passes
through the second electrode 617, a stack of a thin metal film and
a transparent conductive film (e.g., ITO, indium oxide containing
zinc oxide at 2 wt % to 20 wt %, indium tin oxide containing
silicon, or zinc oxide (ZnO)) is preferably used for the second
electrode 617.
[0181] Note that the light-emitting element is formed with the
first electrode 613, the EL layer 616, and the second electrode
617. The light-emitting element has the structure described in any
of Embodiments 1 to 3. In the light-emitting device of this
embodiment, the pixel portion, which includes a plurality of
light-emitting elements, may include both the light-emitting
element described in any of Embodiments 1 to 3 and a light-emitting
element having a different structure.
[0182] The sealing substrate 604 is attached to the element
substrate 610 with the sealing material 605, so that the
light-emitting element 618 is provided in the space 607 surrounded
by the element substrate 610, the sealing substrate 604, and the
sealing material 605. The space 607 may be filled with filler, or
may be filled with an inert gas (such as nitrogen or argon), or the
sealing material 605. It is preferable that the sealing substrate
be provided with a recessed portion and the drying agent 625 be
provided in the recessed portion, in which case deterioration due
to influence of moisture can be suppressed.
[0183] An epoxy-based resin or glass frit is preferably used for
the sealing material 605. It is preferable that such a material do
not transmit moisture or oxygen as much as possible. As the sealing
substrate 604, a glass substrate, a quartz substrate, or a plastic
substrate formed of fiberglass reinforced plastic (FRP), polyvinyl
fluoride (PVF), polyester, acrylic, or the like can be used.
[0184] As described above, the light-emitting device which uses the
light-emitting element described in any of Embodiments 1 to 3 can
be obtained.
[0185] The light-emitting device in this embodiment is fabricated
using the light-emitting element described in any of Embodiments 1
to 3 and thus can have favorable characteristics. Specifically,
since the light-emitting element described in any of Embodiments 1
to 3 has high emission efficiency, the light-emitting device can
have reduced power consumption. In addition, since the
light-emitting element can be driven at low voltage, the
light-emitting device can be driven at low voltage.
[0186] Although an active matrix light-emitting device is described
above in this embodiment, application to a passive matrix
light-emitting device may be carried out. FIGS. 4A and 4B
illustrate a passive matrix light-emitting device manufactured
using the present invention. FIG. 4A is a perspective view of the
light-emitting device, and FIG. 4B is a cross-sectional view taken
along the line X-Y in FIG. 4A. In FIGS. 4A and 4B, over a substrate
951, an EL layer 955 is provided between an electrode 952 and an
electrode 956. An end portion of the electrode 952 is covered with
an insulating layer 953. A partition layer 954 is provided over the
insulating layer 953. The sidewalls of the partition layer 954 are
aslope such that the distance between both sidewalls is gradually
narrowed toward the surface of the substrate. In other words, a
cross section taken along the direction of the short side of the
partition layer 954 is trapezoidal, and the lower side (a side
which is in the same direction as a plane direction of the
insulating layer 953 and in contact with the insulating layer 953)
is shorter than the upper side (a side which is in the same
direction as the plane direction of the insulating layer 953 and
not in contact with the insulating layer 953). The partition layer
954 thus provided can prevent defects in the light-emitting element
due to static electricity or the like. The passive matrix
light-emitting device can also be driven with low power consumption
by including the light-emitting element in any of Embodiments 1 to
3 which can be driven at low voltage. The passive matrix
light-emitting device can also be driven with low power consumption
by including the light-emitting element in any of Embodiments 1 to
3. Further, the passive matrix light-emitting device can have high
reliability by including the light-emitting element in any of
Embodiments 1 to 3.
[0187] For performing full color display, a coloring layer or a
color conversion layer may be provided in a light path through
which light from the light-emitting element passes to the outside
of the light-emitting device. An example of a light-emitting device
in which full color display is achieved with the use of a coloring
layer and the like is illustrated in FIGS. 5A and 5B. In FIG. 5A, a
substrate 1001, a base insulating film 1002, a gate insulating film
1003, gate electrodes 1006, 1007, and 1008, a first interlayer
insulating film 1020, a second interlayer insulating film 1021, a
peripheral portion 1042, a pixel portion 1040, a driver circuit
portion 1041, first electrodes 1024W, 1024R, 1024G, and 1024B of
light-emitting elements, a partition 1025, an EL layer 1028, a
second electrode 1029 of the light-emitting elements, a sealing
substrate 1031, a sealant 1032, and the like are illustrated.
Further, coloring layers (a red coloring layer 1034R, a green
coloring layer 1034G, and a blue coloring layer 1034B) are provided
on a transparent base material 1033. Further, a black layer (a
black matrix) 1035 may be additionally provided. The transparent
base material 1033 provided with the coloring layers and the black
layer is positioned and fixed to the substrate 1001. Note that the
coloring layers and the black layer are covered with an overcoat
layer 1036. In this embodiment, light emitted from part of the
light-emitting layer does not pass through the coloring layers,
while light emitted from the other part of the light-emitting layer
passes through the coloring layers. Since light which does not pass
through the coloring layers is white and light which passes through
any one of the coloring layers is red, blue, or green, an image can
be displayed using pixels of the four colors.
[0188] The above-described light-emitting device is a
light-emitting device having a structure in which light is
extracted from the substrate 1001 side where the TFTs are formed (a
bottom emission structure), but may be a light-emitting device
having a structure in which light is extracted from the sealing
substrate 1031 side (a top emission structure). FIG. 6 is a
cross-sectional view of a light-emitting device having a top
emission structure. In this case, a substrate which does not
transmit light can be used as the substrate 1001. The process up to
the step of forming of a connection electrode which connects the
TFT and the anode of the light-emitting element is performed in a
manner similar to that of the light-emitting device having a bottom
emission structure. Then, a third interlayer insulating film 1037
is formed to cover an electrode 1022. This insulating film may have
a planarization function. The third interlayer insulating film 1037
can be formed using a material similar to that of the second
interlayer insulating film, and can alternatively be formed using
any other known material.
[0189] The first electrodes 1024W, 1024R, 1024G, and 1024B of the
light-emitting elements each serve as an anode here, but may serve
as a cathode. Further, in the case of a light-emitting device
having a top emission structure as illustrated in FIG. 6, the first
electrodes are preferably reflective electrodes. The EL layer 1028
is formed to have a structure similar to the structure described in
any of Embodiments 1 to 3, with which white light emission can be
obtained. As the structure with which white light emission can be
obtained, in the case where two EL layers are used, a structure
with which blue light is obtained from a light-emitting layer in
one of the EL layers and orange light is obtained from a
light-emitting layer of the other of the EL layers; a structure in
which blue light is obtained from a light-emitting layer of one of
the EL layers and red light and green light are obtained from a
light-emitting layer of the other of the EL layers; and the like
can be given. Further, in the case where three EL layers are used,
red light, green light, and blue light are obtained from respective
light-emitting layers, so that a light-emitting element which emits
white light can be obtained. Needless to say, the structure with
which white light emission is obtained is not limited thereto as
long as the structure described in any of Embodiments 1 to 3 is
used.
[0190] The coloring layers are each provided in a light path
through which light from the light-emitting element passes to the
outside of the light-emitting device. In the case of the
light-emitting device having a bottom emission structure as
illustrated in FIG. 5A, the coloring layers 1034R, 1034G, and 1034B
can be provided on the transparent base material 1033 and then
fixed to the substrate 1001. The coloring layers may be provided
between the gate insulating film 1003 and the first interlayer
insulating film 1020 as illustrated in FIG. 5B. In the case of a
top emission structure as illustrated in FIG. 6, sealing can be
performed with the sealing substrate 1031 on which the coloring
layers (the red coloring layer 1034R, the green coloring layer
1034G, and the blue coloring layer 1034B) are provided. The sealing
substrate 1031 may be provided with the black layer (the black
matrix) 1035 which is positioned between pixels. The coloring
layers (the red coloring layer 1034R, the green coloring layer
1034G and the blue coloring layer 1034B) and the black layer (the
black matrix) may be covered with the overcoat layer 1036. Note
that a light-transmitting substrate is used as the sealing
substrate 1031.
[0191] When voltage is applied between the pair of electrodes of
the thus obtained organic light-emitting element, a white
light-emitting region 1044W can be obtained. In addition, by using
the coloring layers, a red light-emitting region 1044R, a blue
light-emitting region 1044B, and a green light-emitting region
1044G can be obtained. The light-emitting device in this embodiment
includes the light-emitting element described in any of Embodiments
1 to 3; thus, a light-emitting device with low power consumption
can be obtained.
[0192] Although an example in which full color display is performed
using four colors of red, green, blue, and white is shown here,
there is no particular limitation and full color display using
three colors of red, green, and blue may be performed.
[0193] This embodiment can be freely combined with any of the other
embodiments.
Embodiment 5
[0194] In this embodiment, an example in which the light-emitting
element described in any of Embodiments 1 to 3 is used for a
lighting device will be described with reference to FIGS. 7A and
7B. FIG. 7B is a top view of the lighting device, and FIG. 7A is a
cross-sectional view taken along the line e-f in FIG. 7B.
[0195] In the lighting device in this embodiment, a first electrode
401 is formed over a substrate 400 which is a support and has a
light-transmitting property. The first electrode 401 corresponds to
the first electrode 101 in Embodiments 1 to 3.
[0196] An auxiliary electrode 402 is provided over the first
electrode 401. Since light emission is extracted through the first
electrode 401 side in the example given in this embodiment, the
first electrode 401 is formed using a material having a
light-transmitting property. The auxiliary electrode 402 is
provided in order to compensate for the low conductivity of the
material having a light-transmitting property, and has a function
of suppressing luminance unevenness in a light emission surface due
to voltage drop caused by the high resistance of the first
electrode 401. The auxiliary electrode 402 is formed using a
material having at least higher conductivity than the material of
the first electrode 401, and is preferably formed using a material
having high conductivity such as aluminum. Note that surfaces of
the auxiliary electrode 402 other than a portion thereof in contact
with the first electrode 401 are preferably covered with an
insulating layer. This is for suppressing light emission over the
upper portion of the auxiliary electrode 402, which cannot be
extracted, and for suppressing a reduction in power efficiency.
Note that a pad 412 for applying a voltage to a second electrode
404 may be formed at the same time as the auxiliary electrode
402.
[0197] An EL layer 403 is formed over the first electrode 401 and
the auxiliary electrode 402. The EL layer 403 has the structure
described in any of Embodiments 1 to 3. Refer to the descriptions
for the structure. Note that the EL layer 403 is preferably formed
to be slightly larger than the first electrode 401 when seen from
above, in which case the EL layer 403 can also serve as an
insulating layer that suppresses a short circuit between the first
electrode 401 and the second electrode 404.
[0198] The second electrode 404 is formed to cover the EL layer
403. The second electrode 404 corresponds to the second electrode
102 in Embodiments 1 to 3 and has a similar structure. In this
embodiment, it is preferable that the second electrode 404 be
formed using a material having high reflectance because light
emission is extracted through the first electrode 401 side. In this
embodiment, the second electrode 404 is connected to the pad 412,
whereby voltage is applied.
[0199] As described above, the lighting device described in this
embodiment includes a light-emitting element including the first
electrode 401, the EL layer 403, and the second electrode 404 (and
the auxiliary electrode 402). Since the light-emitting element is a
light-emitting element with high emission efficiency, the lighting
device in this embodiment can be a lighting device having low power
consumption. Furthermore, since the light-emitting element has high
reliability, the lighting device in this embodiment can be a
lighting device having high reliability.
[0200] The light-emitting element having the above structure is
fixed to a sealing substrate 407 with sealing materials 405 and
sealing is performed, whereby the lighting device is completed. It
is possible to use only one of the sealing materials 405. The inner
sealing material 405 can be mixed with a desiccant which enables
moisture to be adsorbed, increasing reliability.
[0201] When parts of the pad 412, the first electrode 401, and the
auxiliary electrode 402 are extended to the outside of the sealing
materials 405 and 406, the extended parts can serve as external
input terminals. An IC chip 420 mounted with a converter or the
like may be provided over the external input terminals.
[0202] As described above, since the lighting device described in
this embodiment includes the light-emitting element described in
any of Embodiments 1 to 3 as an EL element, the lighting device can
be a lighting device having low power consumption. Further, the
lighting device can be a lighting device which can be driven at low
voltage. Furthermore, the lighting device can be a lighting device
having high reliability.
Embodiment 6
[0203] In this embodiment, examples of electronic devices each
including the light-emitting element described in any of
Embodiments 1 to 3 will be described. The light-emitting element
described in any of Embodiments 1 to 3 has high emission efficiency
and reduced power consumption. As a result, the electronic devices
described in this embodiment can each include a light-emitting
portion having reduced power consumption. In addition, the
electronic devices can be driven at low voltage since the
light-emitting element described in any of Embodiments 1 to 3 can
be driven at low voltage.
[0204] Examples of the electronic device to which the above
light-emitting element is applied include television devices (also
referred to as TV or television receivers), monitors for computers
and the like, cameras such as digital cameras and digital video
cameras, digital photo frames, mobile phones (also referred to as
cell phones or mobile phone devices), portable game machines,
portable information terminals, audio playback devices, large game
machines such as pachinko machines, and the like. Specific examples
of these electronic devices are described below.
[0205] FIG. 8A illustrates an example of a television device. In
the television device, a display portion 7103 is incorporated in a
housing 7101. Here, the housing 7101 is supported by a stand 7105.
Images can be displayed on the display portion 7103, and in the
display portion 7103, the light-emitting elements described in any
of Embodiments 1 to 3 are arranged in a matrix. The light-emitting
elements can have high emission efficiency. Further, the
light-emitting elements can be driven at low voltage. Furthermore,
the light-emitting elements can have a long lifetime. Therefore,
the television device including the display portion 7103 which is
formed using the light-emitting elements can exhibit reduced power
consumption. Further, the television device can be driven at low
voltage. Furthermore, the television device can have high
reliability.
[0206] Operation of the television device can be performed with an
operation switch of the housing 7101 or a separate remote
controller 7110. With operation keys 7109 of the remote controller
7110, channels and volume can be controlled and images displayed on
the display portion 7103 can be controlled. The remote controller
7110 may be provided with a display portion 7107 for displaying
data output from the remote controller 7110.
[0207] Note that the television device is provided with a receiver,
a modem, and the like. With the use of the receiver, general
television broadcasting can be received. Moreover, when the
television device is connected to a communication network with or
without wires via the modem, one-way (from a sender to a receiver)
or two-way (between a sender and a receiver or between receivers)
information communication can be performed.
[0208] FIG. 8B1 illustrates a computer, which includes a main body
7201, a housing 7202, a display portion 7203, a keyboard 7204, an
external connection port 7205, a pointing device 7206, and the
like. Note that this computer is manufactured by arranging
light-emitting elements similar to those described in any of
Embodiments 1 to 3 in a matrix in the display portion 7203. The
computer illustrated in FIG. 8B1 may have a structure illustrated
in FIG. 8B2. The computer illustrated in FIG. 8B2 is provided with
a second display portion 7210 instead of the keyboard 7204 and the
pointing device 7206. The second display portion 7210 has a touch
screen, and input can be performed by operation of images, which
are displayed on the second display portion 7210, with a finger or
a dedicated pen. The second display portion 7210 can also display
images other than the display for input. The display portion 7203
may also have a touch screen. Connecting the two screens with a
hinge can prevent troubles; for example, the screens can be
prevented from being cracked or broken while the computer is being
stored or carried. Note that this computer is manufactured by
arranging the light-emitting elements described in any of
Embodiments 1 to 3 in a matrix in the display portion 7203. The
light-emitting elements can have high emission efficiency.
Therefore, this computer having the display portion 7203 which is
formed using the light-emitting elements can have reduced power
consumption.
[0209] FIG. 8C illustrates a portable game machine having two
housings, a housing 7301 and a housing 7302, which are connected
with a joint portion 7303 so that the portable game machine can be
opened or folded. The housing 7301 incorporates a display portion
7304 including the light-emitting elements described in any of
Embodiments 1 to 3 and arranged in a matrix, and the housing 7302
incorporates a display portion 7305. In addition, the portable game
machine illustrated in FIG. 8C includes a speaker portion 7306, a
recording medium insertion portion 7307, an LED lamp 7308, input
means (an operation key 7309, a connection terminal 7310, a sensor
7311 (a sensor having a function of measuring force, displacement,
position, speed, acceleration, angular velocity, rotational
frequency, distance, light, liquid, magnetism, temperature,
chemical substance, sound, time, hardness, electric field, current,
voltage, electric power, radiation, flow rate, humidity, gradient,
oscillation, odor, or infrared rays), and a microphone 7312), and
the like. Needless to say, the structure of the portable game
machine is not limited to the above as long as the display portion
which includes the light-emitting elements described in any of
Embodiments 1 to 3 and arranged in a matrix is used as at least
either the display portion 7304 or the display portion 7305, or
both, and the structure can include other accessories as
appropriate. The portable game machine illustrated in FIG. 8C has a
function of reading out a program or data stored in a storage
medium to display it on the display portion, and a function of
sharing information with another portable game machine by wireless
communication. Note that functions of the portable game machine
illustrated in FIG. 8C are not limited to them, and the portable
game machine can have various functions. Since the light-emitting
elements used in the display portion 7304 have high emission
efficiency, the portable game machine including the above-described
display portion 7304 can have reduced power consumption. Since each
of the light-emitting elements used in the display portion 7304 can
be driven at low voltage, the portable game machine can also be
driven at low voltage. Furthermore, since the light-emitting
elements used in the display portion 7304 each have a long
lifetime, the portable game machine can have high reliability.
[0210] FIG. 8D illustrates an example of a mobile phone. The mobile
phone is provided with a display portion 7402 incorporated in a
housing 7401, operation buttons 7403, an external connection port
7404, a speaker 7405, a microphone 7406, and the like. Note that
the mobile phone has the display portion 7402 including the
light-emitting elements described in any of Embodiments 1 to 3 and
arranged in a matrix. The light-emitting elements can have high
emission efficiency. Further, the light-emitting elements can be
driven at low voltage. Furthermore, the light-emitting elements can
have a long lifetime. Therefore, the mobile phone including the
display portion 7402 which is found using the light-emitting
elements can have reduced power consumption. Further, the mobile
phone can be driven at low voltage. Furthermore, the mobile phone
can have high reliability.
[0211] When the display portion 7402 of the mobile phone
illustrated in FIG. 8D is touched with a finger or the like, data
can be input into the mobile phone. In this case, operations such
as making a call and creating an e-mail can be performed by
touching the display portion 7402 with a finger or the like.
[0212] There are mainly three screen modes of the display portion
7402. The first mode is a display mode mainly for displaying an
image. The second mode is an input mode mainly for inputting
information such as characters. The third mode is a
display-and-input mode in which two modes of the display mode and
the input mode are combined.
[0213] For example, in the case of making a call or creating an
e-mail, a character input mode mainly for inputting characters is
selected for the display portion 7402 so that characters displayed
on a screen can be input. In this case, it is preferable to display
a keyboard or number buttons on almost the entire screen of the
display portion 7402.
[0214] When a detection device including a sensor for detecting
inclination, such as a gyroscope or an acceleration sensor, is
provided inside the mobile phone, display on the screen of the
display portion 7402 can be automatically changed by determining
the orientation of the mobile phone (whether the mobile phone is
placed horizontally or vertically for a landscape mode or a
portrait mode).
[0215] The screen modes are switched by touch on the display
portion 7402 or operation with the operation buttons 7403 of the
housing 7401. The screen modes can be switched depending on the
kinds of images displayed on the display portion 7402. For example,
when a signal of an image displayed on the display portion is a
signal of moving image data, the screen mode is switched to the
display mode. When the signal is a signal of text data, the screen
mode is switched to the input mode.
[0216] Moreover, in the input mode, when input by touching the
display portion 7402 is not performed for a certain period while a
signal detected by an optical sensor in the display portion 7402 is
detected, the screen mode may be controlled so as to be switched
from the input mode to the display mode.
[0217] The display portion 7402 may function as an image sensor.
For example, an image of a palm print, a fingerprint, or the like
is taken by the display portion 7402 while in touch with the palm
or the finger, whereby personal authentication can be performed.
Further, by providing a backlight or a sensing light source which
emits a near-infrared light in the display portion, an image of a
finger vein, a palm vein, or the like can be taken.
[0218] Note that the structure described in this embodiment can be
combined with any of the structures described in Embodiments 1 to 5
as appropriate.
[0219] As described above, the application range of the
light-emitting device having the light-emitting element described
in any of Embodiments 1 to 3 is wide so that this light-emitting
device can be applied to electronic devices in a variety of fields.
By using the light-emitting element described in any of Embodiments
1 to 3, an electronic device having reduced power consumption can
be obtained.
[0220] FIG. 9 illustrates an example of a liquid crystal display
device using the light-emitting element described in any of
Embodiments 1 to 3 for a backlight. The liquid crystal display
device illustrated in FIG. 9 includes a housing 901, a liquid
crystal layer 902, a backlight unit 903, and a housing 904. The
liquid crystal layer 902 is connected to a driver IC 905. The
light-emitting element described in any of Embodiments 1 to 3 is
used in the backlight unit 903, to which current is supplied
through a terminal 906.
[0221] The light-emitting element described in any of Embodiments 1
to 3 is used for the backlight of the liquid crystal display
device; thus, the backlight can have reduced power consumption. In
addition, the use of the light-emitting element described in any of
Embodiments 1 to 3 enables manufacture of a planar-emission
lighting device and further a larger-area planar-emission lighting
device; therefore, the backlight can be a larger-area backlight,
and the liquid crystal display device can also be a larger-area
device. Furthermore, the light-emitting device using the
light-emitting element described in any of Embodiments 1 to 3 can
be thinner than a conventional one; accordingly, the display device
can also be thinner.
[0222] FIG. 10 illustrates an example in which the light-emitting
element described in any of Embodiments 1 to 3 is used for a table
lamp which is a lighting device. The table lamp illustrated in FIG.
10 includes a housing 2001 and a light source 2002, and the
light-emitting element described in any of Embodiments 1 to 3 is
used for the light source 2002.
[0223] FIG. 11 illustrates an example in which the light-emitting
element described in any of Embodiments 1 to 3 is used for an
indoor lighting device 3001. Since the light-emitting element
described in any of Embodiments 1 to 3 has reduced power
consumption, a lighting device that has reduced power consumption
can be obtained. Further, since the light-emitting element
described in any of Embodiments 1 to 3 can have a large area, the
light-emitting element can be used for a large-area lighting
device. Furthermore, since the light-emitting element described in
any of Embodiments 1 to 3 is thin, the light-emitting element can
be used for a lighting device having a reduced thickness.
[0224] The light-emitting element described in any of Embodiments 1
to 3 can also be used for an automobile windshield or an automobile
dashboard. FIG. 12 illustrates one mode in which the light-emitting
elements described in any of Embodiments 1 to 3 are used for an
automobile windshield and an automobile dashboard. Displays regions
5000 to 5005 each include the light-emitting element described in
any of Embodiments 1 to 3.
[0225] The display regions 5000 and the display region 5001 are
provided in the automobile windshield in which the light-emitting
elements described in any of Embodiments 1 to 3 are incorporated.
The light-emitting element described in any of Embodiments 1 to 3
can be formed into what is called a see-through display device,
through which the opposite side can be seen, by including a first
electrode and a second electrode formed of electrodes having
light-transmitting properties. Such see-through display devices can
be provided even in the automobile windshield, without hindering
the vision. Note that in the case where a transistor for driving or
the like is provided, a transistor having a light-transmitting
property, such as an organic transistor using an organic
semiconductor material or a transistor using an oxide
semiconductor, is preferably used.
[0226] The display region 5002 is provided in a pillar portion in
which the light-emitting elements described in any of Embodiments 1
to 3 are incorporated. The display region 5002 can compensate for
the view hindered by the pillar portion by showing an image taken
by an imaging unit provided in the car body. Similarly, the display
region 5003 provided in the dashboard can compensate for the view
hindered by the car body by showing an image taken by an imaging
unit provided in the outside of the car body, which leads to
elimination of blind areas and enhancement of safety. Showing an
image so as to compensate for the area which a driver cannot see
makes it possible for the driver to confirm safety easily and
comfortably.
[0227] The display region 5004 and the display region 5005 can
provide a variety of kinds of information such as navigation data,
a speedometer, a tachometer, a mileage, a fuel meter, a gearshift
indicator, and air-condition setting. The content or layout of the
display can be changed freely by a user as appropriate. Note that
such information can also be shown by the display regions 5000 to
5003. The display regions 5000 to 5005 can also be used as lighting
devices.
[0228] The light-emitting element described in any of Embodiments 1
to 3 can have high emission efficiency and low power consumption.
Therefore, load on a battery is small even when a number of large
screens such as the display regions 5000 to 5005 are provided,
which provides comfortable use. For that reason, the light-emitting
device and the lighting device each of which includes the
light-emitting element described in any of Embodiments 1 to 3 can
be suitably used as an in-vehicle light-emitting device and an
in-vehicle lighting device.
[0229] FIGS. 13A and 13B illustrate an example of a foldable tablet
terminal. FIG. 13A illustrates the tablet terminal which is
unfolded. The tablet terminal includes a housing 9630, a display
portion 9631a, a display portion 9631b, a display mode switch 9034,
a power switch 9035, a power-saving mode switch 9036, a clasp 9033,
and an operation switch 9038. Note that in the tablet terminal, one
of or both the display portion 9631a and the display portion 9631b
is/are formed using a light-emitting device which includes the
light-emitting element described in any of Embodiments 1 to 3.
[0230] Part of the display portion 9631a can be a touchscreen
region 9632a and data can be input when a displayed operation key
9637 is touched. Although half of the display portion 9631a has
only a display function and the other half has a touchscreen
function, one embodiment of the present invention is not limited to
the structure. The whole display portion 9631a may have a
touchscreen function. For example, a keyboard can be displayed on
the entire region of the display portion 9631a so that the display
portion 9631a is used as a touchscreen, and the display portion
9631b can be used as a display screen.
[0231] Like the display portion 9631a, part of the display portion
9631b can be a touchscreen region 9632b. When a switching button
9639 for showing/hiding a keyboard on the touchscreen is touched
with a finger, a stylus, or the like, the keyboard can be displayed
on the display portion 9631b.
[0232] Touch input can be performed in the touchscreen region 9632a
and the touchscreen region 9632b at the same time.
[0233] The display mode switch 9034 can switch the display between
portrait mode, landscape mode, and the like, and between monochrome
display and color display, for example. With the switch 9036 for
switching to power-saving mode, the luminance of display can be
optimized in accordance with the amount of external light at the
time when the tablet terminal is in use, which is detected with an
optical sensor incorporated in the tablet terminal. The tablet
terminal may include another detection device such as a sensor for
detecting orientation (e.g., a gyroscope or an acceleration sensor)
in addition to the optical sensor.
[0234] Although FIG. 13A illustrates an example in which the
display portion 9631a and the display portion 9631b have the same
display area, one embodiment of the present invention is not
limited to the example. The display portion 9631a and the display
portion 9631b may have different display areas and different
display quality. For example, higher definition images may be
displayed on one of the display portions 9631a and 9631b.
[0235] FIG. 13B illustrates the tablet terminal which is folded.
The tablet terminal in this embodiment includes the housing 9630, a
solar cell 9633, a charge and discharge control circuit 9634, a
battery 9635, and a DC-to-DC converter 9636. Note that FIG. 13B
illustrates an example in which the charge and discharge control
circuit 9634 includes the battery 9635 and the DC-to-DC converter
9636.
[0236] Since the tablet terminal is foldable, the housing 9630 can
be closed when the tablet terminal is not in use. As a result, the
display portion 9631a and the display portion 9631b can be
protected, thereby providing a tablet terminal with high endurance
and high reliability for long-term use.
[0237] The tablet terminal illustrated in FIGS. 13A and 13B can
have other functions such as a function of displaying various kinds
of data (e.g., a still image, a moving image, and a text image), a
function of displaying a calendar, a date, the time, or the like on
the display portion, a touch-input function of operating or editing
the data displayed on the display portion by touch input, and a
function of controlling processing by various kinds of software
(programs).
[0238] The solar cell 9633 provided on a surface of the tablet
terminal can supply power to the touchscreen, the display portion,
a video signal processing portion, or the like. Note that the solar
cell 9633 is preferably provided on one or two surfaces of the
housing 9630, in which case the battery 9635 can be charged
efficiently.
[0239] The structure and operation of the charge and discharge
control circuit 9634 illustrated in FIG. 13B will be described with
reference to a block diagram of FIG. 13C.
[0240] FIG. 13C illustrates the solar cell 9633, the battery 9635,
the DC-to-DC converter 9636, a converter 9638, switches SW1 to SW3,
and a display portion 9631. The battery 9635, the DC-to-DC
converter 9636, the converter 9638, and the switches SW1 to SW3
correspond to the charge and discharge control circuit 9634
illustrated in FIG. 13B.
[0241] First, description is made on an example of the operation in
the case where power is generated by the solar cell 9633 with the
use of external light. The voltage of the power generated by the
solar cell is raised or lowered by the DC-to-DC converter 9636 so
as to be voltage for charging the battery 9635. Then, when power
from the solar cell 9633 is used for the operation of the display
portion 9631, the switch SW1 is turned on and the voltage of the
power is raised or lowered by the converter 9638 so as to be
voltage needed for the display portion 9631. When images are not
displayed on the display portion 9631, the switch SW1 is turned off
and the switch SW2 is turned on so that the battery 9635 is
charged.
[0242] Although the solar cell 9633 is described as an example of a
power generation means, the power generation means is not
particularly limited, and the battery 9635 may be charged by
another power generation means such as a piezoelectric element or a
thermoelectric conversion element (Peltier element). The battery
9635 may be charged by a non-contact power transmission module
capable of performing charging by transmitting and receiving power
wirelessly (without contact), or any of the other charge means used
in combination, and the power generation means is not necessarily
provided.
[0243] One embodiment of the present invention is not limited to
the tablet terminal having the shape illustrated in FIGS. 13A to
13C as long as the display portion 9631a or 9631b is included.
Example 1
[0244] In this example, a method for fabricating a light-emitting
element which corresponds to one embodiment of the present
invention described in any of Embodiments 1 to 3 and the
characteristics thereof will be described. Structural formulae of
organic compounds used in this example are shown below.
##STR00001## ##STR00002## ##STR00003##
[0245] Next, a method for fabricating the light-emitting element in
this example will be described below.
[0246] First, a film of indium oxide-tin oxide containing silicon
oxide (ITSO) was formed over a glass substrate by a sputtering
method, so that the first electrode 101 was formed. The thickness
thereof was 110 nm and the electrode area was 2 mm.times.2 mm.
Here, the first electrode 101 functions as an anode of the
light-emitting element.
[0247] Next, as pretreatment for forming the light-emitting element
over the substrate, UV-ozone treatment was performed for 370
seconds after washing of a surface of the substrate with water and
baking that was performed at 200.degree. C. for one hour.
[0248] After that, the substrate was transferred into a vacuum
evaporation apparatus where the pressure had been reduced to
approximately 10.sup.-4 Pa, and was subjected to vacuum baking at
170.degree. C. for 30 minutes in a heating chamber of the vacuum
evaporation apparatus, and then the substrate was cooled down for
about 30 minutes.
[0249] Then, the substrate over which the first electrode 101 was
formed was fixed to a substrate holder provided in the vacuum
evaporation apparatus so that the surface on which the first
electrode 101 was formed faced downward. The pressure in the vacuum
evaporation apparatus was reduced to about 10.sup.-4 Pa. After
that, over the first electrode 101,
4,4',4''-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:
DBT3P-II) represented by Structural Formula (i) and molybdenum(VI)
oxide were deposited by co-evaporation, so that the hole-injection
layer 111 was formed. The thickness of the hole-injection layer 111
was set to 40 nm, and the weight ratio of DBT3P-II to molybdenum
oxide was adjusted to 4:2. Note that the co-evaporation method
refers to an evaporation method in which evaporation is carried out
from a plurality of evaporation sources at the same time in one
treatment chamber.
[0250] Next, a film of
4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:
BPAFLP) which is represented by Structural Formula (ii) was formed
to a thickness of 20 nm over the hole-injection layer 111 to form
the hole-transport layer 112.
[0251] Over the hole-transport layer 112,
2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2mDBTPDBq-II) represented by Structural Formula
(iii), 4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBA1BP) represented by Structural Formula (iv), and
(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)
(abbreviation: [Ir(tppr).sub.2(dpm)]) represented by Structural
Formula (v) were deposited by co-evaporation to a thickness of 20
nm with a weight ratio of 2mDBTPDBq-II to PCBA1BP and
[Ir(tppr).sub.2(dpm)] being 0.5:0.5:0.05, so that the second
light-emitting layer 113b was formed; after that,
3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:
35DCzPPy) represented by Structural Formula (vi),
3,3'-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP) represented by
Structural Formula (vii), and
tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-.-
kappa.N2]phenyl-.kappa.C}iridium(III) (abbreviation:
[Ir(mpptz-dmp).sub.3]) represented by Structural Formula (viii)
(the compound (1)) were deposited by co-evaporation to a thickness
of 30 nm with a weight ratio of 35DCzPPy to PCCP and
[Ir(mpptz-dmp).sub.3] being 0.5:0.5:0.06, so that the first
light-emitting layer 113a was formed.
[0252] Then, the electron-transport layer 114 was formed over the
light-emitting layer 113 in such a way that a 10-nm-thick film of
2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole
(abbreviation: mDBTBIm-II) represented by Structural formula (ix)
was formed and a 20-nm-thick film of bathophenanthroline
(abbreviation: BPhen) represented by Structural Formula (x) was
formed.
[0253] After the formation of the electron-transport layer 114,
lithium fluoride (LiF) was deposited by evaporation to a thickness
of 1 nm, so that the electron-injection layer 115 was formed.
[0254] Lastly, aluminum was deposited by evaporation to a thickness
of 200 nm to form the second electrode 102 functioning as a
cathode. Thus, a light-emitting element 1 in this example was
fabricated.
[0255] Note that in all the above evaporation steps, evaporation
was performed by a resistance-heating method.
[0256] Table 1 shows an element structure of the light-emitting
element 1 obtained in the above manner.
TABLE-US-00001 TABLE 1 Hole- Hole- Electron- First injection
transport Light- Electron- injection Second electrode layer layer
emitting layer transport layer layer electrode Light-emitting ITSO
DBT3P-II:MoOx BPAFLP * ** mDBTBIm-II Bphen LiF Al element 1 (110
nm) (4:2 40 nm) (20 nm) (10 nm) (20 nm) (1 nm) (200 nm) *
2mDBTPDBq-II:PCBA1BP:[Ir(tppr).sub.2(dpm)] (0.5:0.5:0.05 20 nm) **
35DCzPPy:PCCP:[Ir(mpptz-dmp).sub.3] (0.5:0.5:0.06 30 nm)
[0257] The light-emitting element 1 was sealed using a glass
substrate in a glove box containing a nitrogen atmosphere so as not
to be exposed to the air (specifically, a sealing material was
applied onto an outer edge of the element and heat treatment was
performed at 80.degree. C. for 1 hour at the time of sealing).
[0258] Element characteristics of the light-emitting element were
measured. Note that the measurements were carried out at room
temperature (in an atmosphere kept at 25.degree. C.).
[0259] FIG. 14 shows an emission spectrum of the light-emitting
element 1 which was obtained when a current of 0.1 mA was made to
flow in the light-emitting element 1. FIG. 14 indicates that the
light-emitting element 1 shows an emission spectrum including light
with a wavelength in the blue wavelength range which originates
from [Ir(mpptz-dmp).sub.3] and light with a wavelength in the red
wavelength range which originates from [Ir(tppr).sub.2(dpm)].
[0260] FIG. 15 shows luminance-current efficiency characteristics
of the light-emitting element 1; FIG. 16 shows luminance-external
quantum efficiency characteristics thereof; FIG. 17 shows
voltage-luminance characteristics thereof; and FIG. 18 shows
luminance-power efficiency characteristics thereof. Table 2 shows
main characteristics of the light-emitting element 1 at around 1000
cd/m.sup.2.
TABLE-US-00002 TABLE 2 Current Current Power External Voltage
Current density Chromaticity efficiency efficiency quantum (V) (mA)
(mA/cm.sup.2) (x, y) (cd/A) (lm/W) efficiency (%) Light-emitting
4.6 0.11 2.6 (0.53, 0.36) 36 24 25 element 1
[0261] From the above, the light-emitting element 1 turned out to
have excellent element characteristics. In particular, as can be
seen from FIG. 15 and FIG. 16, the light-emitting element 1 has
extremely high emission efficiency and has a high external quantum
efficiency of 25% at around a practical luminance (1000
cd/m.sup.2). Further, FIG. 17 shows that the light-emitting element
1 has favorable voltage-luminance characteristics, and is driven at
a low voltage. Thus, as is clear from FIG. 18, the light-emitting
element 1 has favorable power efficiency.
[0262] The above shows that the light-emitting element 1
corresponding to one embodiment of the present invention has
excellent element characteristics and provides lights from two
kinds of emission center substances in a good balance.
Example 2
[0263] In this example, a method for fabricating a light-emitting
element which corresponds to one embodiment of the present
invention described in Embodiments 1 to 3 and the characteristics
thereof will be described. Structural formulae of organic compounds
used in this example are shown below. In this example, a
light-emitting element 2 and a light-emitting element 3 in each of
which the light-emitting layer 113 includes the first
light-emitting layer 113a, the second light-emitting layer 113b,
and the third light-emitting layer 113c were fabricated.
##STR00004## ##STR00005## ##STR00006##
[0264] Next, a method for fabricating the light-emitting element 2
in this example will be described below.
[0265] First, a film of indium oxide-tin oxide containing silicon
oxide (ITSO) was formed over a glass substrate by a sputtering
method, so that the first electrode 101 was formed. The thickness
thereof was 110 nm and the electrode area was 2 mm.times.2 mm.
Here, the first electrode 101 functions as an anode of the
light-emitting element.
[0266] Next, as pretreatment for forming the light-emitting element
over the substrate, UV-ozone treatment was performed for 370
seconds after washing of a surface of the substrate with water and
baking that was performed at 200.degree. C. for one hour.
[0267] After that, the substrate was transferred into a vacuum
evaporation apparatus where the pressure had been reduced to
approximately 10.sup.-4 Pa, and was subjected to vacuum baking at
170.degree. C. for 30 minutes in a heating chamber of the vacuum
evaporation apparatus, and then the substrate was cooled down for
about 30 minutes.
[0268] Then, the substrate over which the first electrode 101 was
formed was fixed to a substrate holder provided in the vacuum
evaporation apparatus so that the surface on which the first
electrode 101 was formed faced downward. The pressure in the vacuum
evaporation apparatus was reduced to about 10.sup.-4 Pa. After
that, over the first electrode 101, DBT3P-II and molybdenum(VI)
oxide were deposited by co-evaporation by an evaporation method
using resistance heating, so that the hole-injection layer 111 was
formed. The thickness of the hole-injection layer 111 was set to 40
nm, and the weight ratio of DBT3P-II to molybdenum oxide was
adjusted to 4:2. Note that the co-evaporation method refers to an
evaporation method in which evaporation is carried out from a
plurality of evaporation sources at the same time in one treatment
chamber.
[0269] Next, a film of BPAFLP was formed to a thickness of 20 nm
over the hole-injection layer 111 to form the hole-transport layer
112.
[0270] Over the hole-transport layer 112, 2mDBTPDBq-II, PCBA1BP,
and [Ir(tppr).sub.2(dpm)] were deposited by co-evaporation to a
thickness of 10 nm with a weight ratio of 2mDBTPDBq-II to PCBA1BP
and [Ir(tppr).sub.2(dpm)] being 0.5:0.5:0.05, so that the third
light-emitting layer 113c was formed; then, 2mDBTPDBq-II, PCBA1BP,
and
(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)
(abbreviation: [Ir(tBuppm).sub.2(acac)]) represented by Structural
Formula (xi) were deposited by co-evaporation to a thickness of 5
nm with a weight ratio of 2mDBTPDBq-II to PCBA1BP and
[Ir(tBuppm).sub.2(acac)] being 0.5:0.5:0.05, so that the second
light-emitting layer 113b was formed; after that, 35DCzPPy, PCCP,
and [Ir(mpptz-dmp).sub.3] were deposited by co-evaporation to a
thickness of 30 nm with a weight ratio of 35DCzPPy to PCCP and
[Ir(mpptz-dmp).sub.3] being 0.5:0.5:0.06, so that the first
light-emitting layer 113a was faulted.
[0271] Then, the electron-transport layer 114 was formed over the
light-emitting layer 113 in such a way that a 10-nm-thick film of
mDBTBIm-II was formed and a 20-nm-thick film of BPhen was
formed.
[0272] After the formation of the electron-transport layer 114,
lithium fluoride (LiF) was deposited by evaporation to a thickness
of 1 nm, so that the electron-injection layer 115 was formed.
[0273] Lastly, aluminum was deposited by evaporation to a thickness
of 200 nm to form the second electrode 102 functioning as a
cathode. Thus, the light-emitting element 2 in this example was
fabricated.
[0274] Note that in all the above evaporation steps, evaporation
was performed by a resistance-heating method.
[0275] Next, a method for fabricating the light-emitting element 3
will be described. The light-emitting element 3 was fabricated in
such a manner that the thickness of the second light-emitting layer
113b in the light-emitting element 2 was changed from 5 nm to 10
nm. The other structures are the same as those of the
light-emitting element 2.
[0276] Table 3 shows element structures of the light-emitting
element 2 and the light-emitting element 3 obtained in the above
manner.
TABLE-US-00003 TABLE 3 Hole- Hole- Electron- First injection
transport Light- Electron- injection Second electrode layer layer
emitting layer transport layer layer electrode Light-emitting ITSO
DBT3P-II:MoOx BPAFLP * ** *** mDBTBIm-II Bphen LiF Al element 2
(110 nm) (4:2 40 nm) (20 nm) (10 nm) (20 nm) (1 nm) (200 nm)
Light-emitting ITSO DBT3P-II:MoOx BPAFLP * **** *** mDBTBIm-II
Bphen LiF Al element 3 (110 nm) (4:2 40 nm) (20 nm) (10 nm) (20 nm)
(1 nm) (200 nm) * 2mDBTPDBq-II:PCBA1BP:[Ir(tppr).sub.2(dpm)]
(0.5:0.5:0.05 10 nm) **
2mDBTPDBq-II:PCBA1BP:[Ir(tBuppm).sub.2(acac)] (0.5:0.5:0.05 5 nm)
*** 35DCzPPy:PCCP:[Ir(mpptz-dmp).sub.3] (0.5:0.5:0.06 30 nm) ****
2mDBTPDBq-II:PCBA1BP:[Ir(tBuppm).sub.2(acac)] (0.5:0.5:0.05 10
nm)
[0277] The light-emitting element 2 and the light-emitting element
3 were sealed using a glass substrate in a glove box containing a
nitrogen atmosphere so as not to be exposed to the air
(specifically, a sealing material was applied onto an outer edge of
the element and heat treatment was performed at 80.degree. C. for 1
hour at the time of sealing).
[0278] Element characteristics of the light-emitting elements were
measured. Note that the measurements were carried out at room
temperature (in an atmosphere kept at 25.degree. C.).
[0279] FIG. 19 shows emission spectra of the light-emitting element
2 and the light-emitting element 3 which were obtained when a
current of 0.1 mA was made to flow in the light-emitting element 2
and the light-emitting element 3. FIG. 19 indicates that the
light-emitting element 2 and the light-emitting element 3 show
emission spectra each including light with a wavelength in the blue
wavelength range which originates from [Ir(mpptz-dmp).sub.3], light
with a wavelength in the green wavelength range which originates
from [Ir(tBuppm).sub.2(acac)], and light with a wavelength in the
red wavelength range which originates from [Ir(tppr).sub.2(dpm)].
In particular, the light-emitting element 2 emits light that meets
the standards defined by JIS.
[0280] FIG. 20 shows luminance-current efficiency characteristics
of the light-emitting element 2 and the light-emitting element 3;
FIG. 21 shows luminance-external quantum efficiency characteristics
thereof; FIG. 22 shows voltage-luminance characteristics thereof;
and FIG. 23 shows luminance-power efficiency characteristics
thereof. Table 4 shows main characteristics of the light-emitting
element 2 and the light-emitting element 3 at around 1000
cd/m.sup.2.
TABLE-US-00004 TABLE 4 Current Current Power External Voltage
Current density Chromaticity efficiency efficiency quantum (V) (mA)
(mA/cm.sup.2) (x, y) (cd/A) (lm/W) efficiency (%) Light-emitting
4.6 0.083 2.1 (0.46, 0.44) 47 32 22 element 2 Light-emitting 4.6
0.084 2.1 (0.44, 0.46) 52 36 22 element 3
[0281] From the above, the light-emitting element 2 and the
light-emitting element 3 turned out to have excellent element
characteristics. In particular, as can be seen from FIG. 20 and
FIG. 21, the light-emitting element 2 and the light-emitting
element 3 each have extremely high emission efficiency and have a
high external quantum efficiency exceeding 20% at around a
practical luminance (1000 cd/m.sup.2). Further, FIG. 22 shows that
the light-emitting element 2 and the light-emitting element 3 have
favorable voltage-luminance characteristics, and are driven at a
low voltage. Thus, as is clear from FIG. 23, the light-emitting
element 2 and the light-emitting element 3 have favorable power
efficiency.
[0282] The above shows that the light-emitting element 2 and the
light-emitting element 3 each corresponding to one embodiment of
the present invention have favorable element characteristics and
provide lights from three kinds of emission center substances in a
good balance.
Example 3
[0283] In this example, a method for fabricating a light-emitting
element which corresponds to one embodiment of the present
invention described in any of Embodiments 1 to 3 and the
characteristics thereof will be described. Structural formulae of
organic compounds used in this example are shown below. In this
example, a light-emitting element 4 in which the light-emitting
layer 113 includes the first light-emitting layer 113a, the second
light-emitting layer 113b, and the third light-emitting layer 113c
was fabricated.
##STR00007## ##STR00008## ##STR00009##
[0284] Next, a method for fabricating the light-emitting element 4
in this example will be described below.
[0285] First, a film of indium oxide-tin oxide containing silicon
oxide (ITSO) was formed over a glass substrate by a sputtering
method, so that the first electrode 101 was formed. The thickness
thereof was 110 nm and the electrode area was 2 mm.times.2 mm.
Here, the first electrode 101 functions as an anode of the
light-emitting element.
[0286] Next, as pretreatment for forming the light-emitting element
over the substrate, UV-ozone treatment was performed for 370
seconds after washing of a surface of the substrate with water and
baking that was performed at 200.degree. C. for one hour.
[0287] After that, the substrate was transferred into a vacuum
evaporation apparatus where the pressure had been reduced to
approximately 10.sup.-4 Pa, and was subjected to vacuum baking at
170.degree. C. for 30 minutes in a heating chamber of the vacuum
evaporation apparatus, and then the substrate was cooled down for
about 30 minutes.
[0288] Then, the substrate over which the first electrode 101 was
formed was fixed to a substrate holder provided in the vacuum
evaporation apparatus so that the surface on which the first
electrode 101 was formed faced downward. The pressure in the vacuum
evaporation apparatus was reduced to about 10.sup.-4 Pa. After
that, over the first electrode 101, DBT3P-II and molybdenum(VI)
oxide were deposited by co-evaporation by an evaporation method
using resistance heating, so that the hole-injection layer 111 was
formed. The thickness of the hole-injection layer 111 was set to 40
nm, and the weight ratio of DBT3P-II to molybdenum oxide was
adjusted to 4:2. Note that the co-evaporation method refers to an
evaporation method in which evaporation is carried out from a
plurality of evaporation sources at the same time in one treatment
chamber.
[0289] Next, a film of
4,4'-di(1-naphthyl)-4''-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(PCBNBB)] which is represented by Structural Formula (xii) was
formed to a thickness of 20 nm over the hole-injection layer 111 to
form the hole-transport layer 112.
[0290] Over the hole-transport layer 112,
2-[3'-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline
(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula
(xiii), PCBNBB, and [Ir(tppr).sub.2(dpm)] were deposited by
co-evaporation to a thickness of 20 nm with a weight ratio of
2mDBTBPDBq-II to PCBNBB and [Ir(tppr).sub.2(dpm)] being
0.5:0.5:0.05, so that the third light-emitting layer 113c was
formed; then, 2mDBTBPDBq-II, PCBNBB, and [Ir(tBuppm).sub.2(acac)]
were deposited by co-evaporation to a thickness of 10 nm with a
weight ratio of 2mDBTBPDBq-II to PCBNBB and
[Ir(tBuppm).sub.2(acac)] being 0.5:0.5:0.05, so that the second
light-emitting layer 113b was formed; after that, 35DCzPPy, PCCP,
and [Ir(mpptz-dmp).sub.3] were deposited by co-evaporation to a
thickness of 30 nm with a weight ratio of 35DCzPPy to PCCP and
[Ir(mpptz-dmp).sub.3] being 0.7:0.3:0.06, so that the first
light-emitting layer 113a was formed.
[0291] Then, the electron-transport layer 114 was formed over the
light-emitting layer 113 in such a way that a 10-nm-thick film of
35DCzPPy was formed and a 20-nm-thick film of BPhen was formed.
[0292] After the formation of the electron-transport layer 114,
lithium fluoride (LiF) was deposited by evaporation to a thickness
of 1 nm, so that the electron-injection layer 115 was formed.
[0293] Lastly, aluminum was deposited by evaporation to a thickness
of 200 nm to form the second electrode 102 functioning as a
cathode. Thus, the light-emitting element 4 in this example was
fabricated.
[0294] Note that in all the above evaporation steps, evaporation
was performed by a resistance-heating method.
[0295] Table 5 shows an element structure of the light-emitting
element 4 obtained in the above manner.
TABLE-US-00005 TABLE 5 Hole- Hole- Electron- First injection
transport Light- Electron- injection Second electrode layer layer
emitting layer transport layer layer electrode Light-emitting ITSO
DBT3P-II:MoOx PCBNBB * ** *** 35DCzPPy Bphen LiF Al element 4 (110
nm) (4:2 40 nm) (20 nm) (10 nm) (20 nm) (1 nm) (200 nm) *
2mDBTBPDBq-II:PCBNBB:[Ir(tppr).sub.2(dpm)] (0.5:0.5:0.05 20 nm) **
2mDBTBPDBq-II:PCBNBB:[Ir(tBuppm).sub.2(acac)] (0.5:0.5:0.05 10 nm)
*** 35DCzPPy:PCCP:[Ir(mpptz-dmp).sub.3] (0.7:0.3:0.06 30 nm)
[0296] The light-emitting element 4 was sealed using a glass
substrate in a glove box containing a nitrogen atmosphere so as not
to be exposed to the air (specifically, a sealing material was
applied onto an outer edge of the element and heat treatment was
performed at 80.degree. C. for 1 hour at the time of sealing).
[0297] Element characteristics of the light-emitting element were
measured. Note that the measurements were carried out at room
temperature (in an atmosphere kept at 25.degree. C.).
[0298] FIG. 24 shows an emission spectrum of the light-emitting
element 4 which was obtained when a current of 0.1 mA was made to
flow in the light-emitting element 4. FIG. 24 indicates that the
light-emitting element 4 shows an emission spectrum including light
with a wavelength in the blue wavelength range which originates
from [Ir(mpptz-dmp).sub.3], light with a wavelength in the green
wavelength range which originates from [Ir(tBuppm).sub.2(acac)],
and light with a wavelength in the red wavelength range which
originates from [Ir(tppr).sub.2(dpm)].
[0299] FIG. 25 shows luminance-current efficiency characteristics
of the light-emitting element 4; FIG. 26 shows luminance-external
quantum efficiency characteristics thereof; FIG. 27 shows
voltage-luminance characteristics thereof; and FIG. 28 shows
luminance-power efficiency characteristics thereof. Table 6 shows
main characteristics of the light-emitting element 4 at around 1000
cd/m.sup.2.
TABLE-US-00006 TABLE 6 Current Current Power External Voltage
Current density Chromaticity efficiency efficiency quantum (V) (mA)
(mA/cm.sup.2) (x, y) (cd/A) (lm/W) efficiency (%) Light-emitting
4.4 0.092 2.3 (0.53, 0.41) 38 27 23 element 4
[0300] From the above, the light-emitting element 4 turned out to
have excellent element characteristics. In particular, as can be
seen from FIG. 25 and FIG. 26, the light-emitting element 4 has
extremely high emission efficiency and has a high external quantum
efficiency exceeding 20% at around a practical luminance (1000
cd/m.sup.2). Further, FIG. 27 shows that the light-emitting element
4 has favorable voltage-luminance characteristics, and is driven at
a low voltage. Thus, as is clear from FIG. 28, the light-emitting
element 4 has favorable power efficiency.
[0301] The above shows that the light-emitting element 4
corresponding to one embodiment of the present invention has
favorable element characteristics and provides lights from three
kinds of emission center substances in a good balance.
[0302] Further, FIG. 29 shows the results of a reliability test
under conditions where the initial luminance was 3000 cd/m.sup.2
and the current density was constant. FIG. 29 shows a change in
normalized luminance with an initial luminance of 100%. The results
show that a decrease in luminance over driving time of the
light-emitting element 4 is small, and thus the light-emitting
element 4 has favorable reliability.
Reference Example 1
[0303] The triplet excitation energies of 35DCzPPy, PCCP,
2mDBTPDBq-II, PCBA1BP, 2mDBTBPDBq-II, and PCBNBB used for the
light-emitting elements in the above examples were measured. Note
that the triplet excitation energies were measured in such a manner
that phosphorescent emission of each substance was measured and a
phosphorescence wavelength was converted into electron volt. In the
measurement, each substance was irradiated with excitation light
with a wavelength of 325 nm and the measurement temperature was 10
K. Note that in measuring an energy level, calculation from an
absorption wavelength is more accurate than calculation from an
emission wavelength. However, here, absorption of the triplet
excitation energy was extremely low and difficult to measure; thus,
the triplet excitation energy was measured by measuring a peak
wavelength located on the shortest wavelength side in a
phosphorescence spectrum. For that reason, a few errors may be
included in the measured values.
[0304] FIG. 30, FIG. 31, FIG. 32, FIG. 33, FIG. 34, and FIG. 35
show measured phosphorescence. Table 7 shows the measurement
results. As apparent from these results, the triplet excitation
energies of 35DCzPPy and PCCP used for the first light-emitting
layer are higher than those of 2mDBTPDBq-II, PCBA1BP, 2mDB
IBPDBq-II, and PCBNBB used for the second light-emitting layer or
the third light-emitting layer.
TABLE-US-00007 TABLE 7 Triplet excitation energies of substances
35DCzPPy 2.74 eV PCCP 2.64 eV 2mDBTPDBq-II 2.40 eV PCBA1BP 2.46 eV
2mDBTBPDBq-II 2.41 eV PCBNBB 2.21 eV
Reference Example 2
[0305] A synthetic example of an organometallic complex
(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)
(another name:
bis[2-(6-tert-butyl-4-pyrimidinyl-.kappa.N3)phenyl-.kappa.C](2,4-pentaned-
ionato-.kappa..sup.2O,O')iridium(III)) (abbreviation:
[Ir(tBuppm).sub.2(acac)]), which is used in the above examples,
will be described. The structure of [Ir(tBuppm).sub.2(acac)] is
shown below.
##STR00010##
Step 1: Synthesis of 4-tert-butyl-6-phenylpyrimidine (abbreviation:
HtBuppm)
[0306] First, 22.5 g of 4,4-dimethyl-1-phenylpentane-1,3-dione and
50 g of formamide were put into a recovery flask equipped with a
reflux pipe, and the air in the flask was replaced with nitrogen.
This reaction container was heated, so that the reacted solution
was refluxed for 5 hours. After that, this solution was poured into
an aqueous solution of sodium hydroxide, and an organic layer was
extracted with dichloromethane. The obtained organic layer was
washed with water and saturated saline, and dried with magnesium
sulfate. The solution after drying was filtered. The solvent of
this solution was distilled off, and then the obtained residue was
purified by silica gel column chromatography using hexane and ethyl
acetate as a developing solvent in a volume ratio of 10:1, so that
a pyrimidine derivative HtBuppm (colorless oily substance, yield of
14%) was obtained. A synthesis scheme of Step 1 is shown below.
##STR00011##
Step 2: Synthesis of
di-.mu.-chloro-bis[bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)](ab-
breviation: [Ir(tBuppm).sub.2Cl].sub.2)
[0307] Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.49 g of
HtBuppm obtained in Step 1, and 1.04 g of iridium chloride hydrate
(IrCl.sub.3.H.sub.2O) were put into a recovery flask equipped with
a reflux pipe, and the air in the flask was replaced with argon.
After that, irradiation with microwaves (2.45 GHz, 100 W) was
performed for 1 hour to cause a reaction. The solvent was distilled
off, and then the obtained residue was suction-filtered and washed
with ethanol, so that a dinuclear complex
[Ir(tBuppm).sub.2Cl].sub.2 (yellow green powder, yield of 73%) was
obtained. A synthesis scheme of Step 2 is shown below.
##STR00012##
Step 3: Synthesis of
(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)
(abbreviation: [ft(tBuppm).sub.2(acac)])
[0308] Further, 40 mL of 2-ethoxyethanol, 1.61 g of the dinuclear
complex [Ir(tBuppm).sub.2Cl].sub.2 obtained in Step 2, 0.36 g of
acetylacetone, and 1.27 g of sodium carbonate were put into a
recovery flask equipped with a reflux pipe, and the air in the
flask was replaced with argon. After that, irradiation with
microwaves (2.45 GHz, 120 W) was performed for 60 minutes to cause
a reaction. The solvent was distilled off, and the obtained residue
was suction-filtered with ethanol and washed with water and
ethanol. This solid was dissolved in dichloromethane, and the
mixture was filtered through a filter aid in which Celite (produced
by Wako Pure Chemical Industries, Ltd., Catalog No. 531-16855),
alumina, and Celite were stacked in this order. The solvent was
distilled off, and the obtained solid was recrystallized with a
mixed solvent of dichloromethane and hexane, so that the objective
substance was obtained as yellow powder (yield of 68%). A synthesis
scheme of Step 3 is shown below.
##STR00013##
[0309] An analysis result by nuclear magnetic resonance
spectrometry (.sup.1H NMR) of the yellow powder obtained in Step 3
is described below. These results revealed that the organometallic
complex Ir(tBuppm).sub.2(acac) was obtained.
[0310] .sup.1H NMR. .delta. (CDCl.sub.3): 1.50 (s, 18H), 1.79 (s,
6H), 5.26 (s, 1H), 6.33 (d, 2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.70
(d, 2H), 7.76 (s, 2H), 9.02 (s, 2H).
Reference Example 3
[0311] In this reference example, a synthesis method of
tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-.-
kappa.N2]phenyl-.kappa.C}iridium(III) (abbreviation:
[Ir(mpptz-dmp).sub.3]), which is used in the above example, will be
described. A structure of [Ir(mpptz-dmp).sub.3] is shown below.
##STR00014##
Step 1: Synthesis of N-benzoyl-N'-2-methylbenzoylhydrazide
[0312] First, 15.0 g (110.0 mmol) of benzoylhydrazine and 75 ml of
N-methyl-2-pyrrolidinone (NMP) were put into a 300-ml three-neck
flask and stirred while being cooled with ice. To this mixed
solution, a mixed solution of 17.0 g (110.0 mmol) of o-toluoyl
chloride and 15 ml of N-methyl-2-pyrrolidinone (NMP) was slowly
added dropwise. After the addition, the mixture was stirred at room
temperature for 24 hours. After reaction for the predetermined
time, this reacted solution was slowly added to 500 ml of water, so
that a white solid was precipitated. The precipitated solid was
subjected to ultrasonic cleaning in which water and 1M hydrochloric
acid were used alternately. Then, ultrasonic cleaning using hexane
was performed, so that 19.5 g of a white solid of
N-benzoyl-N'-2-methylbenzoylhydrazide was obtained in a yield of
70%. A synthesis scheme of Step 1 is shown below.
##STR00015##
Step 2: Synthesis of
N-[1-chloro-1-(2-methylphenyl)methylidene]-N'-[1-chloro-(1-phenyl)methyli-
dene]hydrazine
[0313] Next, 12.0 g (47.2 mmol) of
N-benzoyl-N'-2-methylbenzoylhydrazide obtained in Step 1 and 200 ml
of toluene were put into a 500-ml three-neck flask. To this mixed
solution, 19.4 g (94.4 mmol) of phosphorus pentachloride was added
and the mixture was heated and stirred at 120.degree. C. for 6
hours. After reaction for the predetermined time, the reacted
solution was slowly poured into 200 ml of water and the mixture was
stirred for 1 hour. After the stirring, an organic layer and an
aqueous layer were separated, and the organic layer was washed with
water and a saturated aqueous solution of sodium hydrogen
carbonate. After the washing, the organic layer was dried with
anhydrous magnesium sulfate. The magnesium sulfate was removed from
this mixture by gravity filtration, and the filtrate was
concentrated; thus, 12.6 g of a brown liquid of
N-[1-chloro-1-(2-methylphenyl)methylidene]-N'-[1-chloro-(1-phen-
yl)methylidene]hydr azine was obtained in a yield of 92%. A
synthesis scheme of Step 2 is shown below.
##STR00016##
Step 3: Synthesis of
3-(2-methylphenyl)-4-(2,6-dimethylphenyl)-5-phenyl-4H-1,2,4-triazole
(abbreviation: Hmpptz-dmp)
[0314] First, 12.6 g (43.3 mmol) of
N-[1-chloro-1-(2-methylphenyl)methylidene]-N'-[1-chloro-(1-phenyl)methyli-
dene]hydr azine obtained in Step 2, 15.7 g (134.5 mmol) of
2,6-dimethylaniline, and 100 ml of N,N-dimethylaniline were put
into a 500-ml recovery flask and heated and stirred at 120.degree.
C. for 20 hours. After reaction for the predetermined time, this
reacted solution was slowly added to 200 ml of 1N hydrochloric
acid. Dichloromethane was added to this solution and an objective
substance was extracted to an organic layer. The obtained organic
layer was washed with water and an aqueous solution of sodium
hydrogen carbonate, and was dried with magnesium sulfate. The
magnesium sulfate was removed by gravity filtration, and the
obtained filtrate was concentrated to give a black liquid. This
liquid was purified by silica gel column chromatography. A mixed
solvent of ethyl acetate and hexane in a ratio of 1:5 was used as a
developing solvent. The obtained fraction was concentrated to give
a white solid. This solid was recrystallized with ethyl acetate to
give 4.5 g of a white solid of Hmpptz-dmp in a yield of 31%. A
synthesis scheme of Step 3 is shown below.
##STR00017##
Step 4: Synthesis of [Ir(mpptz-dmp).sub.3]
[0315] Then, 2.5 g (7.4 mmol) of Hmpptz-dmp, which was the ligand
obtained in Step 3, and 0.7 g (1.5 mmol) of
tris(acetylacetonato)iridium(III) were put into a container for
high-temperature heating, and degasification was carried out. The
mixture in the reaction container was heated and stirred at
250.degree. C. for 48 hours under Ar flow. After reaction for the
predetermined time, the obtained solid was washed with
dichloromethane, and an insoluble green solid was obtained by
suction filtration. This solid was dissolved in toluene and
filtered through a stack of alumina and Celite. The obtained
fraction was concentrated to give a green solid. This solid was
recrystallized with toluene, so that 0.8 g of a green powder was
obtained in a yield of 45%. A synthesis scheme of Step 4 is shown
below.
##STR00018##
[0316] An analysis result by nuclear magnetic resonance
(.sup.1H-NMR) spectroscopy of the green powder obtained in Step 4
is described below. The result revealed that [Ir(mpptz-dmp).sub.3]
was obtained by the synthesis method.
[0317] .sup.1H-NMR. .delta. (toluene-d8): 1.82 (s, 9H), 1.90 (s,
9H), 2.64 (s, 9H), 6.56-6.62 (m, 9H), 6.67-6.75 (m, 9H), 6.82-6.88
(m, 3H), 6.91-6.97 (t, 3H), 7.00-7.12 (m, 6H), 7.63-7.67 (d,
3H).
[0318] This application is based on Japanese Patent Application
serial no. 2012-177795 filed with Japan Patent Office on Aug. 10,
2012, the entire contents of which are hereby incorporated by
reference.
* * * * *